Phase Angle Of Entrainment Research Articles

Differences in post-chronic jet lag parameters in male and female mice

ABSTRACT Chronic jet lag has deleterious effects on many physiological parameters. Most simulations of chronic jet lag consist of repeated advances only, while real-life conditions may differ. We compared number of transients, activity (α) and rest (ρ), and robustness of rhythm by autocorrelation in male and female AKR mice; before, during, and after simulated chronic jet lag (CJL); and phase angle of entrainment and accuracy before and after CJL, using alternate 9-h advances and delays of the light-dark schedule. Females exhibited no scalloping. Compared to pre-CJL values, α was significantly more in females post-CJL, indicating α decompression. The levels of activity increased during and after CJL, particularly in females, indicating behavioural feedback on the circadian rhythm. Robustness of rhythm (autocorrelation coefficient, and amplitude) was significantly suppressed during CJL in both males and females. Females took more transients than males to re-entrain to the final LD shift at the end of CJL. More transients were needed to re-entrain at the end of CJL, compared to a single shift. The study suggests that animals undergoing CJL incorporating alternating advances and delays are able to avoid α compression associated with jet lag, and also that females are detrimentally affected more than males. Abbreviations: SCN: Suprachiasmatic Nuclei; CJL: Chronic Jet Lag; LD: Light-dark

RDNA-13. VALIDATION OF BEHAVIORAL ANALYSIS ACROSS AGE IN A MOUSE MODEL FOR FUTURE INVESTIGATION OF RADIATION-INDUCED HYPERSOMNOLENCE (RIH) IN PRIMARY BRAIN TUMOR (PBT) PATIENTS

Cranial radiation is part of standard PBT treatment with patient age recognized to impact response and toxicity. RIH is one of the most common toxicities. We have reported an association between circadian gene variants and RIH susceptibility. No rodent model for RIH has been validated. The purpose of this study is to evaluate the impact of age on activity and circadian behavior to inform our novel mouse model of RIH. Ethovision XT Software and Phenotyper cages wereused to continually video animal behavior and generate general activity data. Male young (6wk, n=3) or old (18mo, n=3) C57BL/6 mice were individually housed. Mice were monitored for 10 days under 12:12hr light:dark conditions. The data was examined for activity and circadian parameters. Older mice had lower total activity levels (-36%) than younger controls, with greatly decreased levels during the active period (-49%) but increased levels during the sleep period (49%). Actograms of general activity binned in 10 min intervals demonstrated a dramatic impact of age on circadian rhythms. Older mice displayed a dampened amplitude of activity rhythms (-36%; t= 4.1, p<0.01) and higher phase angle of entrainment (40.3 ± 25.8min) compared to younger mice (-1.11 ± 0min). Earlier activity onset in older animals increased the total activity period (alpha) by 7% (t=2.9, p<0.05). Both general activity and circadian data collected with Ethovision XT provided clear validation of the technique. CONCLUSIONS: Activity, sleep and circadian rhythms are impacted by irradiation in patients. A good behavioral model to observe these changes has not yet been developed in rodents. Here we demonstrate a reliable system to collect/ analyze both activity and circadian parameters. Our study demonstrates a clear difference in all parameters at age extremes for control animals, a factor that warrants inclusion in our RIH model designed to recapitulate the human experience.

The Phosphorylation of CREB at Serine 133 Is a Key Event for Circadian Clock Timing and Entrainment in the Suprachiasmatic Nucleus.

Within the suprachiasmatic nucleus (SCN)-the locus of the master circadian clock- transcriptional regulation via the CREB/CRE pathway is implicated in the functioning of the molecular clock timing process, and is a key conduit through which photic input entrains the oscillator. One event driving CRE-mediated transcription is the phosphorylation of CREB at serine 133 (Ser133). Indeed, numerous reporter gene assays have shown that an alanine point mutation in Ser133 reduces CREB-mediated transcription. Here, we sought to examine the contribution of Ser133 phosphorylation to the functional role of CREB in SCN clock physiology in vivo. To this end, we used a CREB knock-in mouse strain, in which Ser133 was mutated to alanine (S/A CREB). Under a standard 12 h light-dark cycle, S/A CREB mice exhibited a marked alteration in clock-regulated wheel running activity. Relative to WT mice, S/A CREB mice had highly fragmented bouts of locomotor activity during the night phase, elevated daytime activity, and a delayed phase angle of entrainment. Further, under free-running conditions, S/A CREB mice had a significantly longer tau than WT mice and reduced activity amplitude. In S/A CREB mice, light-evoked clock entrainment, using both Aschoff type 1 and 6 h "jet lag" paradigms, was markedly reduced relative to WT mice. S/A CREB mice exhibited attenuated transcriptional drive, as assessed by examining both clock-gated and light-evoked gene expression. Finally, SCN slice culture imaging detected a marked disruption in cellular clock phase synchrony following a phase-resetting stimulus in S/A CREB mice. Together, these data indicate that signaling through CREB phosphorylation at Ser133 is critical for the functional fidelity of both SCN timing and entrainment.

Chronic BMY7378 treatment alters behavioral circadian rhythms.

The mammalian circadian clock is synchronized to the day:night cycle by light. Serotonin modulates the circadian effects of light, with agonists inhibiting response to light and antagonists enhancing responses to light. A special class of serotonergic compounds, the mixed 5-HT1A agonist/antagonists, potentiates light-induced phase advances by up to 400% when administered acutely. In this study, we examine the effects of one of these mixed 5-HT1A agonist/antagonists, BMY7378, when administered chronically. Thirty adult male hamsters were administered either vehicle or BMY7378 via surgically implanted osmotic mini pumps over a period of 28days. In a light:dark cycle, chronic BMY7378 advanced the phase angle of entrainment, prolonged the duration of the active phase and attenuated the amplitude of the wheel-running rhythm during the early night. In constant darkness, chronic treatment with BMY7378 significantly attenuated light-induced phase advances, but had no significant effect on light-induced phase delays. Non-photic phase shifts to daytime administration of a 5-HT1A/7 agonist were also attenuated by chronic BMY7378 treatment. qRT-PCR analysis revealed that chronic BMY7378 treatment upregulated mRNA for 5-HT1A and 5-HT1B receptors in the hypothalamus and downregulated mRNA for 5-HT1A and monoamine oxidase-A in the brainstem. These results highlight adaptive changes of serotonin receptors in the brain to chronic treatment with BMY7378 and link such up- and downregulation to changes in important circadian parameters. Such long-term changes to the circadian system should be considered when patients are treated chronically with drugs that alter serotonergic function.

0710 REPRODUCIBILITY OF CIRCADIAN VARIABLES (MEQ, MSF, DLMO, PHASE ANGLE, AND CIRCADIAN PERIOD) OVER MONTHS

We conducted two studies that were similar for the first 10 days (nature.com/articles/srep08381, nature.com/articles/srep36716), and determined each subject’s morningness-eveningness score (MEQ), Mid-sleep on Free Days (MSF) from the MCTQ, baseline dim light melatonin onset (DLMO), phase angle of entrainment, and free-running circadian period. Ten African-Americans (6 women, 4 men) and 8 European-Americans (2 women, 6 men) participated in both studies separated by 9 to 33 months (mean ± SD = 16 ± 7). They were 32.2 ± 6.7 years old at the first study. The purpose of this report is to examine the reproducibility of these circadian variables. Subjects slept in the lab on a fixed 8-h sleep schedule similar to their usual sleep schedule for 4 days, followed by a circadian phase assessment with 30 min saliva samples to calculate baseline DLMO. Phase angle was the interval from DLMO to bedtime. There were also 3 days of ultradian LD cycles producing forced desynchrony. Circadian period was determined from phase assessments before and after the days of ultradian LD cycles. For each circadian variable, we made scatter plots with identical x and y axes. Lines of unity showed when the variable was exactly the same in both studies. Lines parallel to and on each side of the line of unit showed how much the variable differed between the studies. We also calculated Pearson correlations for each variable. The MEQ score differed by less than 10 points between the two studies; MSF by 1 h or less, except for 2 subjects, baseline DLMO by 1 h or less except for 3 subjects, phase angle by 2 h or less, and circadian period by 0.3 h or less except for 2 subjects. All correlations were significant (p≤.0001), MEQ r=.85, MSF r=.78, baseline DLMO r=.81, phase angle r=.80, circadian period r=.78. A longer time between the two studies did not make the variables more different. In this small sample, there were no differences between men and women or between European and African-Americans in the stability of these variables. Circadian parameters were relatively stable over months. NIHR01NR007677.

0701 CIRCADIAN TAU DIFFERENCES IN BIOLOGICAL, BEHAVIOURAL AND SLEEPINESS RHYTHMS IN DELAYED SLEEP-WAKE PHASE DISORDER AND NON-24-HOUR SLEEP-WAKE RHYTHM DISORDER PATIENTS

In this study we investigated biological, sleepiness and behavioural rhythm period lengths (i.e., taus) of Delayed Sleep-Wake Phase Disorder (DSWPD), Non-24-Hour Sleep-Wake Rhythm Disorder (N24SWD) patients and healthy control sleepers. We also ran cross-correlation analyses between different rhythm variables to examine phase angle of entrainment. The aim was to explore if behavioural rhythms, in addition to the biological circadian rhythms contribute to misalignments of sleep timing symptomatic of DSWPD and N24SWD. Twenty-six DSWPD participants who met diagnostic criteria (17m, 9f, age: 21.85 ± 4.97 years) and 18 controls (10m, 8f, age: 23.72 ± 5.10 years) participated in an 80-hour modified constant routine. Additionally, 4 full-sighted patients (3m, 1f, age: 25.75 ± 4.99 years) were diagnosed with N24SWD and included as a discrete study group. A forced-desynchrony ultradian protocol of 1-hour ‘days’ in dim light, controlled conditions alternated 20-minute sleep opportunities with 40-minute enforced wakefulness. Subjective sleepiness ratings were recorded prior to every sleep opportunity and median reaction time (vigilance) was measured hourly. Amount of sleep obtained (sleep propensity) was derived from 20-minute sleep opportunities to quantify hourly objective sleepiness. Hourly core body temperature was recorded and salivary melatonin assayed to measure endogenous circadian rhythms. Rhythm data were curved using the 2-component cosine model. DSWPD and N24SWD patients had significantly longer melatonin and temperature taus compared to controls. There were no significant tau differences between groups as measured by subjective sleepiness, sleep propensity and vigilance rhythms. However, DSWPD patients showed a greater interval between maximum sleep propensity and minimum core body temperature. Their sleep propensity rhythms lagged core temperature rhythms by an hour more compared to controls’ sleep propensity and core temperature rhythms. The findings provide further evidence that delayed circadian rhythms in DSWPD may result from larger phase angles between core body temperature and sleep propensity. This interval may result in later sleep timing in DSWPD patients relative to their circadian timing thus masking their light exposure during a time that is critical to phase-advancing the circadian system. This project was funded by the Australian Research Council.

C-terminal phosphorylation regulates the kinetics of a subset of melanopsin-mediated behaviors in mice

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin and mediate several non-image-forming visual functions, including circadian photoentrainment and the pupillary light reflex (PLR). ipRGCs act as autonomous photoreceptors via the intrinsic melanopsin-based phototransduction pathway and as a relay for rod/cone input via synaptically driven responses. Under low light intensities, where only synaptically driven rod/cone input activates ipRGCs, the duration of the ipRGC response will be determined by the termination kinetics of the rod/cone circuits. Little is known, however, about the termination kinetics of the intrinsic melanopsin-based phototransduction pathway and its contribution to several melanopsin-mediated behaviors. Here, we show that C-terminal phosphorylation of melanopsin determines the recovery kinetics of the intrinsic melanopsin-based photoresponse in ipRGCs, the duration of the PLR, and the speed of reentrainment. In contrast, circadian phase alignment and direct effects of light on activity (masking) are not influenced by C-terminal phosphorylation of melanopsin. Electrophysiological measurements demonstrate that expression of a virally encoded melanopsin lacking all C-terminal phosphorylation sites (C terminus phosphonull) leads to a prolonged intrinsic light response. In addition, mice expressing the C terminus phosphonull in ipRGCs reentrain faster to a delayed light/dark cycle compared with mice expressing virally encoded WT melanopsin; however, the phase angle of entrainment and masking were indistinguishable. Importantly, a sustained PLR in the phosphonull animals is only observed at brighter light intensities that activate melanopsin phototransduction, but not at dimmer light intensities that activate only the rod/cone pathway. Taken together, our results highlight how the kinetics of the melanopsin photoresponse differentially regulate distinct light-mediated behaviors.

Circadian Kinetics of Cell Cycle Progression in Adult Neurogenic Niches of a Diurnal Vertebrate.

The circadian system may regulate adult neurogenesis via intracellular molecular clock mechanisms or by modifying the environment of neurogenic niches, with daily variation in growth factors or nutrients depending on the animal's diurnal or nocturnal lifestyle. In a diurnal vertebrate, zebrafish, we studied circadian distribution of immunohistochemical markers of the cell division cycle (CDC) in 5 of the 16 neurogenic niches of adult brain, the dorsal telencephalon, habenula, preoptic area, hypothalamus, and cerebellum. We find that common to all niches is the morning initiation of G1/S transition and daytime S-phase progression, overnight increase in G2/M, and cycle completion by late night. This is supported by the timing of gene expression for critical cell cycle regulators cyclins D, A2, and B2 and cyclin-dependent kinase inhibitor p20 in brain tissue. The early-night peak in p20, limiting G1/S transition, and its phase angle with the expression of core clock genes, Clock1 and Per1, are preserved in constant darkness, suggesting intrinsic circadian patterns of cell cycle progression. The statistical modeling of CDC kinetics reveals the significant circadian variation in cell proliferation rates across all of the examined niches, but interniche differences in the magnitude of circadian variation in CDC, S-phase length, phase angle of entrainment to light or clock, and its dispersion. We conclude that, in neurogenic niches of an adult diurnal vertebrate, the circadian modulation of cell cycle progression involves both systemic and niche-specific factors.SIGNIFICANCE STATEMENT This study establishes that in neurogenic niches of an adult diurnal vertebrate, the cell cycle progression displays a robust circadian pattern. Common to neurogenic niches located in diverse brain regions is daytime progression of DNA replication and nighttime mitosis, suggesting systemic regulation. Differences between neurogenic niches in the phase and degree of S-phase entrainment to the clock suggest additional roles for niche-specific regulatory mechanisms. Understanding the circadian regulation of adult neurogenesis can help optimize the timing of therapeutic approaches in patients with brain traumas or neurodegenerative disorders and preserve neural stem cells during cytostatic cancer therapies.

Variability of behavioral chronotypes of 16 mammalian species under controlled conditions

Human chronotypes (differences in preference for early or late rising each day) have been extensively studied in recent years, but no attempt has been made to compare human chronotypes with the chronotypes of other animal species. We evaluated behavioral chronotypes in 16 mammalian species along a body size gradient of five orders of magnitude (from mice to cattle). Individuals of all species were studied under a 12L:12D photoperiod in a thermoneutral environment with food and water available at all times. Rhythms of locomotor activity were analyzed for onset time, acrophase, and robustness. Neither of these rhythmic parameters was significantly related to body size, but onset time and acrophase varied considerably from species to species, thus characterizing diurnal and nocturnal species. Chronotype spreads ranged from less than an hour in sheep to almost 24h in cats, thus extending both below and above the human chronotype spread of 6h. The variability of chronotype (as quantified by the standard deviation of group means) was much larger between species than within species and also larger between individuals of a species than within individuals on consecutive days. These results help situate the matter of human chronotypes within the broader context of variability in the phase angle of entrainment of circadian rhythms in animals.

Chronotype and Improved Sleep Efficiency Independently Predict Depressive Symptom Reduction after Group Cognitive Behavioral Therapy for Insomnia.

Cognitive behavioral therapy for insomnia (CBT-I) has been shown to improve both sleep and depressive symptoms, but predictors of depression outcome following CBT-I have not been well examined. This study investigated how chronotype (i.e., morningness-eveningness trait) and changes in sleep efficiency (SE) were related to changes in depressive symptoms among recipients of CBT-I. Included were 419 adult insomnia outpatients from a sleep disorders clinic (43.20% males, age mean ± standard deviation = 48.14 ± 14.02). All participants completed the Composite Scale of Morningness and attended at least 4 sessions of a 6-session group CBT-I. SE was extracted from sleep diary; depressive symptoms were assessed using the Beck Depression Inventory (BDI) prior to (Baseline), and at the end (End) of intervention. Multilevel structural equation modeling revealed that from Baseline to End, SE increased and BDI decreased significantly. Controlling for age, sex, BDI, and SE at Baseline, stronger evening chronotype and less improvement in SE significantly and uniquely predicted less reduction in BDI from Baseline to End. Chronotype did not predict improvement in SE. In an insomnia outpatient sample, SE and depressive symptoms improved significantly after a CBT-I group intervention. All chronotypes benefited from sleep improvement, but those with greater eveningness and/or less sleep improvement experienced less reduction in depressive symptom severity. This suggests that evening preference and insomnia symptoms may have distinct relationships with mood, raising the possibility that the effect of CBT-I on depressive symptoms could be enhanced by assessing and addressing circadian factors.

Effects of lighting condition on circadian behavior in 5-HT1A receptor knockout mice

Serotonin (5-HT) is an important regulator of the mammalian circadian system, and has been implicated in modulating entrained and free-running rhythms, as well as photic and non-photic phase shifting. In general, 5-HT appears to oppose the actions of light on the circadian system of nocturnal rodents. As well, 5-HT mediates, at least in part, some non-photic responses. The 5-HT1A, 1B and 7 receptors regulate these acute responses to zeitgebers. 5-HT also regulates some entrained and free-running properties of the circadian clock. The receptors that contribute to these phenomena have not been fully examined. Here, we use 5-HT1A receptor knockout (KO) mice to examine the response of the mouse circadian system to a variety of lighting conditions, including a normal light–dark cycle (LD), T-cycles, phase advanced LD cycles, constant darkness (DD), constant light (LL) and a 6hour dark pulse starting at CT5. Relative to wildtype mice, the 5-HT1A receptor KO mice have lower levels of activity during the first 8h of the night/subjective night in LD and LL, later activity onsets on transient days during re-entrainment, shorter free-running periods in LL when housed with wheels, and smaller phase shifts to dark pulses. No differences were noted in activity levels during DD, alpha under any light condition, free-running period in DD, or phase angle of entrainment in LD. While the 5-HT1A receptor plays an important role in regulating photic and non-photic phase shifting, its contribution to entrained and free-running properties of the circadian clock is relatively minor.

A longitudinal assessment of sleep timing, circadian phase, and phase angle of entrainment across human adolescence.

The aim of this descriptive analysis was to examine sleep timing, circadian phase, and phase angle of entrainment across adolescence in a longitudinal study design. Ninety-four adolescents participated; 38 (21 boys) were 9–10 years (“younger cohort”) and 56 (30 boys) were 15–16 years (“older cohort”) at the baseline assessment. Participants completed a baseline and then follow-up assessments approximately every six months for 2.5 years. At each assessment, participants wore a wrist actigraph for at least one week at home to measure self-selected sleep timing before salivary dim light melatonin onset (DLMO) phase – a marker of the circadian timing system – was measured in the laboratory. Weekday and weekend sleep onset and offset and weekend-weekday differences were derived from actigraphy. Phase angles were the time durations from DLMO to weekday sleep onset and offset times. Each cohort showed later sleep onset (weekend and weekday), later weekend sleep offset, and later DLMO with age. Weekday sleep offset shifted earlier with age in the younger cohort and later in the older cohort after age 17. Weekend-weekday sleep offset differences increased with age in the younger cohort and decreased in the older cohort after age 17. DLMO to sleep offset phase angle narrowed with age in the younger cohort and became broader in the older cohort. The older cohort had a wider sleep onset phase angle compared to the younger cohort; however, an age-related phase angle increase was seen in the younger cohort only. Individual differences were seen in these developmental trajectories. This descriptive study indicated that circadian phase and self-selected sleep delayed across adolescence, though school-day sleep offset advanced until no longer in high school, whereupon offset was later. Phase angle changes are described as an interaction of developmental changes in sleep regulation interacting with psychosocial factors (e.g., bedtime autonomy).

29.: Disturbances in melatonin secretion and circadian sleep-wake regulation in Parkinson’s disease

Using salivary dim light melatonin onset (DLMO) and actigraphy, the current study sought to identify whether Parkinson’s disease (PD) patients demonstrate circadian disturbance compared to controls. Additionally, this study investigates whether circadian disturbances represent a disease related process or may be attributed to dopaminergic therapy. Twenty-nine patients with PD were divided into unmedicated and medicated groups and were compared to 27 controls. All participants had neurologic assessment and underwent 14 days of actigraphy to establish habitual sleep onset time (HSO). DLMO time and area under the melatonin curve (AUC) were calculated from salivary melatonin sampling. The phase angle of entrainment was calculated by subtracting DLMO from HSO. Overnight polysomnography was performed to determine sleep architecture. DLMO and HSO were not different across the groups. However, the phase angle of entrainment was more than twice as long in the medicated PD group compared to unmedicated PD (U = 35.5, p = 0.002) and was greater than 50% longer than controls (U = 130.0, p = 0.021). The medicated PD group showed more than double the melatonin AUC compared to the unmedicated group (U = 31, p = 0.001) and controls (U = 87, p = 0.001). There was no difference on these measures comparing unmedicated PD and controls. In PD, dopaminergic treatment profoundly increases the secretion of melatonin. This study reports no difference in circadian phase and HSO between groups. However patients treated with dopaminergic therapy unexpectedly showed a delayed sleep onset relative to DLMO, suggesting dopaminergic therapy in PD results in an uncoupling of circadian and sleep regulation.

The serotonergic anxiolytic buspirone attenuates circadian responses to light.

Serotonergic drugs modify circadian responses to light, with agonists attenuating and some partial agonists or antagonists potentiating photic phase shifts. The anxiolytic buspirone is a 5-HT1A receptor partial agonist. Given that buspirone is used therapeutically to manage generalised anxiety disorder, it would be useful to understand if and how this drug may modify circadian responses to light, not only to help manage side effects, but also to examine its potential use as a chronobiotic. Here we examined behavioral and molecular responses to phase-shifting light in mice and hamsters treated with buspirone. Phase advances to late subjective night light pulses in hamsters and wildtype mice were significantly attenuated by buspirone. 5-HT1A receptor knockout mice exhibited potentiated photic phase shifts when pretreated with buspirone. In wildtype mice, the attenuated phase shifts were accompanied by increased cFos expression in the suprachiasmatic nucleus, whereas potentiated phase shifts in knockouts were accompanied by increased phosphorylation of extracellular signal-regulated kinase (ERK) and cyclic AMP response element-binding protein (CREB), and decreased cFos expression. Attenuated photic phase shifts in buspirone-treated hamsters were accompanied by decreased phosphorylation of ERK and CREB. Chronic buspirone treatment decreased the amplitude of wheel-running rhythms, lengthened the duration of the active phase and advanced the phase angle of entrainment. Buspirone administration at midday produced non-photic phase advances in wildtype but not 5-HT1A receptor knockout mice. These findings suggest that buspirone affected the circadian system in a manner similar to the 5-HT1A/7 agonist (±)-8-Hydroxy-2-dipropylaminotetralin hydrobromide, primarily through the 5-HT1A receptor, and suggest that therapeutic use of buspirone to manage anxiety may impact circadian function.

Disturbances in melatonin secretion and circadian sleep–wake regulation in Parkinson disease

ObjectiveUsing salivary dim light melatonin onset (DLMO) and actigraphy, our study sought to determine if Parkinson disease (PD) patients demonstrate circadian disturbance compared to healthy controls. Additionally, our study investigated if circadian disturbances represent a disease-related process or may be attributed to dopaminergic therapy. MethodsTwenty-nine patients with PD were divided into unmedicated and medicated groups and were compared to 27 healthy controls. All participants underwent neurologic assessment and 14days of actigraphy to establish habitual sleep-onset time (HSO). DLMO time and area under the melatonin curve (AUC) were calculated from salivary melatonin sampling. The phase angle of entrainment was calculated by subtracting DLMO from HSO. Overnight polysomnography (PSG) was performed to determine sleep architecture. ResultsDLMO and HSO were not different across the groups. However, the phase angle of entrainment was more than twice as long in the medicated PD group compared to the unmedicated PD group (U=35.5; P=.002) and was more than 50% longer than controls (U=130.0; P=.021). The medicated PD group showed more than double the melatonin AUC compared to the unmedicated group (U=31; P=0.001) and controls (U=87; P=.001). There was no difference in these measures comparing unmedicated PD and controls. ConclusionsIn PD dopaminergic treatment profoundly increases the secretion of melatonin. Our study reported no difference in circadian phase and HSO between groups. However, PD patients treated with dopaminergic therapy unexpectedly showed a delayed sleep onset relative to DLMO, suggesting dopaminergic therapy in PD results in an uncoupling of circadian and sleep regulation.

Circadian Phase and Its Relationship to Nighttime Sleep in Toddlers

Circadian phase and its relation to sleep are increasingly recognized as fundamental factors influencing human physiology and behavior. Dim light melatonin onset (DLMO) is a reliable marker of the timing of the circadian clock, which has been used in experimental, clinical, and descriptive studies in the past few decades. Although DLMO and its relationship to sleep have been well documented in school-aged children, adolescents, and adults, very little is known about these processes in early childhood. The purpose of this study was 1) to describe circadian phase and phase angles of entrainment in toddlers and 2) to examine associations between DLMO and actigraphic measures of children's nighttime sleep. Participants were 45 healthy toddlers aged 30 to 36 months (33.5 ± 2.2 months; 21 females). After sleeping on a parent-selected schedule for 5 days (assessed with actigraphy and diaries), children participated in an in-home DLMO assessment involving the collection of saliva samples every 30 minutes for 6 hours. Average bedtime was 2015 ± 0036 h, average sleep onset time was 2043 ± 0043 h, average midsleep time was 0143 ± 0038 h, and average wake time was 0644 ± 0042 h. Average DLMO was 1929 ± 0051 h, with a 3.5-hour range. DLMO was normally distributed; however, the distribution of the bedtime, sleep onset time, and midsleep phase angles of entrainment were skewed. On average, DLMO occurred 47.8 ± 47.6 minutes (median = 39.4 minutes) before bedtime, 74.6 ± 48.0 minutes (median = 65.4 minutes) before sleep onset time, 6.2 ± 0.7 hours (median = 6.1 hours) before midsleep time, and 11.3 ± 0.7 hours before wake time. Toddlers with later DLMOs had later bedtimes (r = 0.46), sleep onset times (r = 0.51), midsleep times (r = 0.66), and wake times (r = 0.65) (all p < 0.001). Interindividual differences in toddlers' circadian phase are large and associated with their sleep timing. The early DLMOs of toddlers indicate a maturational delay in the circadian timing system between early childhood and adolescence. These findings are a first step in describing the fundamental properties of the circadian system in toddlers and have important implications for understanding the emergence of sleep problems and the consequences of circadian misalignment in early childhood.

Methylphenidate Modifies the Motion of the Circadian Clock

People with attention-deficit/hyperactivity disorder (ADHD) often experience sleep problems, and these are frequently exacerbated by the methylphenidate they take to manage their ADHD symptoms. Many of the changes to sleep are consistent with a change in the underlying circadian clock. The present study was designed to determine if methylphenidate alone could alter properties of the circadian clock. Young male mice were examined in light-dark cycles and in constant darkness and recordings were performed on behavioral activity, sleep, and electrical activity in the suprachiasmatic nucleus (SCN) of freely moving mice. Methylphenidate in the drinking water (0.08%) significantly increased activity in the mid-to-late night, and led to a delay in the onset of activity and sleep relative to the light-dark cycle. While locomotor levels returned to baseline after treatment ended, the phase angle of entrainment required at least a week to return to baseline levels. In constant darkness, the free-running period of both wheel-running and general locomotor rhythms was lengthened by methylphenidate. When the treatment ended, the free-running period either remained stable or only partially reverted to baseline levels. Methylphenidate also altered the electrical firing rate rhythms in the SCN. It induced a delay in the trough of the rhythm, an increment in rhythm amplitude, and a reduction in rhythm variability. These observations suggest that methylphenidate alters the underlying circadian clock. The observed changes are consistent with clock alterations that would promote sleep-onset insomnia.

Human phase response curve to a 1 h pulse of bright white light

The phase resetting response of the human circadian pacemaker to light depends on the timing of exposure and is described by a phase response curve (PRC). The current study aimed to construct a PRC for a 1 h exposure to bright white light (∼8000 lux) and to compare this PRC to a <3 lux dim background light PRC. These data were also compared to a previously completed 6.7 h bright white light PRC and a <15 lux dim background light PRC constructed under similar conditions. Participants were randomized for exposure to 1 h of either bright white light (n=18) or <3 lux dim background light (n=18) scheduled at 1 of 18 circadian phases. Participants completed constant routine (CR) procedures in dim light (<3 lux) before and after the light exposure to assess circadian phase. Phase shifts were calculated as the difference in timing of dim light melatonin onset (DLMO) during pre- and post-stimulus CRs. Exposure to 1 h of bright white light induced a Type 1 PRC with a fitted peak-to-trough amplitude of 2.20 h. No discernible PRC was observed in the <3 lux dim background light PRC. The fitted peak-to-trough amplitude of the 1 h bright light PRC was ∼40% of that for the 6.7 h PRC despite representing only 15% of the light exposure duration, consistent with previous studies showing a non-linear duration–response function for the effects of light on circadian resetting.

Photic Resetting and Entrainment in CLOCK-Deficient Mice

Mice lacking the CLOCK protein have a relatively subtle circadian phenotype, including a slightly shorter period in constant darkness, differences in phase resetting after 4-hour light pulses in the early and late night, and a variably advanced phase angle of entrainment in a light-dark (LD) cycle. The present series of experiments was conducted to more fully characterize the circadian phenotype of Clock(-/-) mice under various lighting conditions. A phase-response curve (PRC) to 4-hour light pulses in free-running mice was conducted; the results confirm that Clock(-/-) mice exhibit very large phase advances after 4-hour light pulses in the late subjective night but have relatively normal responses to light at other phases. The abnormal shape of the PRC to light may explain the tendency of CLOCK-deficient mice to begin activity before lights-out when housed in a 12-hour light:12-hour dark lighting schedule. To assess this relationship further, Clock(-/-) and wild-type control mice were entrained to skeleton lighting cycles (1L:23D and 1L:10D:1L:12D). Comparing entrainment under the 2 types of skeleton photoperiods revealed that exposure to 1-hour light in the morning leads to a phase advance of activity onset (expressed the following afternoon) in Clock(-/-) mice but not in the controls. Constant light typically causes an intensity-dependent increase in circadian period in mice, but this did not occur in CLOCK-deficient mice. The failure of Clock(-/-) mice to respond to the period-lengthening effect of constant light likely results from the increased functional impact of light falling in the phase advance zone of the PRC. Collectively, these experiments reveal that alterations in the response of CLOCK-deficient mice to light in several paradigms are likely due to an imbalance in the shape of the PRC to light.

Exposure to Room Light before Bedtime Suppresses Melatonin Onset and Shortens Melatonin Duration in Humans

Millions of individuals habitually expose themselves to room light in the hours before bedtime, yet the effects of this behavior on melatonin signaling are not well recognized. We tested the hypothesis that exposure to room light in the late evening suppresses the onset of melatonin synthesis and shortens the duration of melatonin production. In a retrospective analysis, we compared daily melatonin profiles in individuals living in room light (<200 lux) vs. dim light (<3 lux). Healthy volunteers (n = 116, 18-30 yr) were recruited from the general population to participate in one of two studies. Participants lived in a General Clinical Research Center for at least five consecutive days. Individuals were exposed to room light or dim light in the 8 h preceding bedtime. Melatonin duration, onset and offset, suppression, and phase angle of entrainment were determined. Compared with dim light, exposure to room light before bedtime suppressed melatonin, resulting in a later melatonin onset in 99.0% of individuals and shortening melatonin duration by about 90 min. Also, exposure to room light during the usual hours of sleep suppressed melatonin by greater than 50% in most (85%) trials. These findings indicate that room light exerts a profound suppressive effect on melatonin levels and shortens the body's internal representation of night duration. Hence, chronically exposing oneself to electrical lighting in the late evening disrupts melatonin signaling and could therefore potentially impact sleep, thermoregulation, blood pressure, and glucose homeostasis.