Endogenous Circadian Component Research Articles

Keeping an eye on circadian time in clinical research and medicine.

Daily rhythms are observed in humans and almost all other organisms. Most of these observed rhythms reflect both underlying endogenous circadian rhythms and evoked responses from behaviours such as sleep/wake, eating/fasting, rest/activity, posture changes and exercise. For many research and clinical purposes, it is important to understand the contribution of the endogenous circadian component to these observed rhythms. The goal of this manuscript is to provide guidance on best practices in measuring metrics of endogenous circadian rhythms in humans and promote the inclusion of circadian rhythms assessments in studies of health and disease. Circadian rhythms affect all aspects of physiology. By specifying minimal experimental conditions for studies, we aim to improve the quality, reliability and interpretability of research into circadian and daily (i.e., time-of-day) rhythms and facilitate the interpretation of clinical and translational findings within the context of human circadian rhythms. We describe protocols, variables and analyses commonly used for studying human daily rhythms, including how to assess the relative contributions of the endogenous circadian system and other daily patterns in behaviours or the environment. We conclude with recommendations for protocols, variables, analyses, definitions and examples of circadian terminology. Although circadian rhythms and daily effects on health outcomes can be challenging to distinguish in practice, this distinction may be important in many clinical settings. Identifying and targeting the appropriate underlying (patho)physiology is a medical goal. This review provides methods for identifying circadian effects to aid in the interpretation of published work and the inclusion of circadian factors in clinical research and practice.

Tryptophan-enriched cereal intake improves nocturnal sleep, melatonin, serotonin, and total antioxidant capacity levels and mood in elderly humans

Melatonin and serotonin rhythms, which exhibit a close association with the endogenous circadian component of sleep, are attenuated with increasing age. This decrease seems to be linked to sleep alterations in the elderly. Chrononutrition is a field of chronobiology that establishes the principle of consuming foodstuffs at times of the day when they are more useful for health, improving, therefore, biorhythms and physical performance. Our aim was to analyze whether the consumption of cereals enriched with tryptophan, the precursor of both serotonin and melatonin, may help in the reconsolidation of the sleep/wake cycle and counteract depression and anxiety in 35 middle-aged/elderly (aged 55-75 year) volunteers in a simple blind assay. Data were collected for 3 weeks according to the following schedule: The control week participants consumed standard cereals (22.5 mg tryptophan in 30 g cereals per dose) at breakfast and dinner; for the treatment week, cereals enriched with a higher dose of tryptophan (60 mg tryptophan in 30 g cereals per dose) were eaten at both breakfast and dinner; the posttreatment week volunteers consumed their usual diet. Each participant wore a wrist actimeter that logged activity during the whole experiment. Urine was collected to analyze melatonin and serotonin urinary metabolites and to measure total antioxidant capacity. The consumption of cereals containing the higher dose in tryptophan increased sleep efficiency, actual sleep time, immobile time, and decreased total nocturnal activity, sleep fragmentation index, and sleep latency. Urinary 6-sulfatoxymelatonin, 5-hydroxyindoleacetic acid levels, and urinary total antioxidant capacity also increased respectively after tryptophan-enriched cereal ingestion as well as improving anxiety and depression symptoms. Cereals enriched with tryptophan may be useful as a chrononutrition tool for alterations in the sleep/wake cycle due to age.

Photic and non-photic effects on the daily activity pattern of Mongolian gerbils

The paper analyses the daily activity pattern of Mongolian gerbils with and without access to a running wheel. To evaluate the synchronizing and the masking effects of light, experiments were performed under different photoperiods (L:D = 14:10 h and 10:14 h), and light and dark pulses were applied at different phases of the day–night cycle. In order to get a more direct estimate of the central pacemaker of the circadian system, the body temperature rhythm was investigated via implanted transmitters. Without access to a running wheel, the daily activity pattern was bimodal. One peak occurred in the first half of the light time, the other one around the light–dark transition. Also, the gerbils were more active during the light phase as compared to the dark phase. After unlocking the running wheel, the gerbils were active mainly during the dark time. The activity peak in the first half of the light phase remained, the second one shifted by a phase delay into the dark time. These results were found under both LD-regimens. Light during the night nearly completely suppressed running wheel activity, switching off the light during the day time induced wheel running. Whereas wheel running was clearly affected by light and dark pulses, the general activity was not. The body temperature rhythm also shows two peaks, with the second one being bigger and coinciding with the endogenous component of the circadian body temperature rhythm. It was found around light-off. After unlocking the running wheel, the maximum of the body temperature rhythm shifted to the night. This was not primarily a consequence of the changed activity pattern as shown by means of purification analysis. Removing the direct effects of motor activity led to a body temperature curve that could be described by a cosine function, and the delay shift was found also for the purified data, a better estimate of the endogenous circadian component. The wheel-associated increase in nocturnality is not only due to masking effects of wheel-running activity on the body temperature and activity rhythms. It also involves clock-related processes. Changes in the phase preference may serve as an adaptation mechanism to the changes in the animal's natural environment.

Role of melatonin in the regulation of human circadian rhythms and sleep.

The circadian rhythm of pineal melatonin is the best marker of internal time under low ambient light levels. The endogenous melatonin rhythm exhibits a close association with the endogenous circadian component of the sleep propensity rhythm. This has led to the idea that melatonin is an internal sleep "facilitator" in humans, and therefore useful in the treatment of insomnia and the readjustment of circadian rhythms. There is evidence that administration of melatonin is able: (i) to induce sleep when the homeostatic drive to sleep is insufficient; (ii) to inhibit the drive for wakefulness emanating from the circadian pacemaker; and (iii) induce phase shifts in the circadian clock such that the circadian phase of increased sleep propensity occurs at a new, desired time. Therefore, exogenous melatonin can act as soporific agent, a chronohypnotic, and/or a chronobiotic. We describe the role of melatonin in the regulation of sleep, and the use of exogenous melatonin to treat sleep or circadian rhythm disorders.

Getting through to circadian oscillators: why use constant routines?

Overt 24-h rhythmicity is composed of both exogenous and endogenous components, reflecting the product of multiple (periodic) feedback loops with a core pacemaker at their center. Researchers attempting to reveal the endogenous circadian (near 24-h) component of rhythms commonly conduct their experiments under constant environmental conditions. However, even under constant environmental conditions, rhythmic changes in behavior, such as food intake or the sleep-wake cycle, can contribute to observed rhythmicity in many physiological and endocrine variables. Assessment of characteristics of the core circadian pacemaker and its direct contribution to rhythmicity in different variables, including rhythmicity in gene expression, may be more reliable when such periodic behaviors are eliminated or kept constant across all circadian phases. This is relevant for the assessment of the status of the circadian pacemaker in situations in which the sleep-wake cycle or food intake regimes are altered because of external conditions, such as in shift work or jet lag. It is also relevant for situations in which differences in overt rhythmicity could be due to changes in either sleep oscillatory processes or circadian rhythmicity, such as advanced or delayed sleep phase syndromes, in aging, or in particular clinical conditions. Researchers studying human circadian rhythms have developed constant routine protocols to assess the status of the circadian pacemaker in constant behavioral and environmental conditions, whereas this technique is often thought to be unnecessary in the study of animal rhythms. In this short review, the authors summarize constant routine methodology and what has been learned from constant routines and argue that animal and human circadian rhythm researchers should (continue to) use constant routines as a step on the road to getting through to central and peripheral circadian oscillators in the intact organism.

REPEATED ASSESSMENT OF THE ENDOGENOUS 24-HOUR PROFILE OF BLOOD PRESSURE UNDER CONSTANT ROUTINE*

The impact of environmental and behavioral factors on the 24-h profile of blood pressure (BP) has been well established. Various attempts have been made to control these exogenous factors, in order to investigate a possible endogenous circadian variation of BP. Recently, we reported the results of the first environmentally and behaviorally controlled laboratory study with 24-h recordings of BP and heart rate (HR) during maintained wakefulness. In this constant-routine study, a pronounced endogenous circadian rhythm of HR was found, but circadian variation of BP was absent. This result suggested that the circadian rhythm of BP observed in earlier controlled studies, with sleep allowed, was evoked by the sleep–wake cycle as opposed to the endogenous circadian pacemaker. In order to verify our previous finding during maintained wakefulness, we repeated the experiment five times with six normotensive, healthy young subjects. Statistical analyses of the hourly measurements of BP and HR confirmed the replicable presence of an endogenous circadian rhythm of HR, as well as the consistent absence of an endogenous circadian variation of BP. Thus, this study provided additional evidence that the 24-h profile of BP—as observed under normal circumstances—is the sole result of environmental and behavioral factors such as the occurrence of sleep, and has no endogenous circadian component. (Chronobiology International, 18(1), 85–98, 2001)

CONTRIBUTION OF CIRCADIAN PHYSIOLOGY AND SLEEP HOMEOSTASIS TO AGE-RELATED CHANGES IN HUMAN SLEEP

The circadian pacemaker and sleep homeostasis play pivotal roles in vigilance state control. It has been hypothesized that age-related changes in the human circadian pacemaker, as well as sleep homeostatic mechanisms, contribute to the hallmarks of age-related changes in sleep, that is, earlier wake time and reduced sleep consolidation. Assessments of circadian parameters in healthy young (∼20–30 years old) and older people (∼65–75 years old)—in the absence of the confounding effects of sleep, changes in posture, and light exposure—have demonstrated that an earlier wake time in older people is accompanied by about a 1h advance of the rhythms of core body temperature and melatonin. In addition, older people wake up at an earlier circadian phase of the body temperature and plasma melatonin rhythm. The amplitude of the endogenous circadian component of the core body temperature rhythm assessed during constant routine and forced desynchrony protocols is reduced by 20–30% in older people. Recent assessments of the intrinsic period of the human circadian pacemaker in the absence of the confounding effects of light revealed no age-related reduction of this parameter in both sighted and blind individuals. Wake maintenance and sleep initiation are not markedly affected by age except that sleep latencies are longer in older people when sleep initiation is attempted in the early morning. In contrast, major age-related reductions in the consolidation and duration of sleep occur at all circadian phases. Sleep of older people is particularly disrupted when scheduled on the rising limb of the temperature rhythm, indicating that the sleep of older people is more susceptible to arousal signals genernpated by the circadian pacemaker. Sleep-homeostatic mechanisms, as assayed by the sleep-deprivation–induced increase of EEG slow-wave activity (SWA), are operative in older people, although during both baseline sleep and recovery sleep SWA in older people remains at lower levels. The internal circadian phase advance of awakening, as well as the age-related reduction in sleep consolidation, appears related to an age-related reduction in the promotion of sleep by the circadian pacemaker during the biological night in combination with a reduced homeostatic pressure for sleep. Early morning light exposure associated with this advance of awakening in older people could reinforce the advanced circadian phase. Quantification of the interaction between sleep homeostasis and circadian rhythmicity contributes to understanding age-related changes in sleep timing and quality. (Chronobiology International, 17(3), 285–311, 2000)

Beyond the three-process model of alertness: estimating phase, time on shift, and successive night effects.

This paper starts by summarizing the development and refinement of the additive three-process model of alertness first published by Folkard and Akerstedt in 1987. It reviews some of the successes that have been achieved by the model in not only predicting variations in subjective alertness on abnormal sleep-wake schedules but also in accounting for objective measures of sleep latency and duration. Nevertheless, predictions derived from the model concerning alertness on different shifts, and over successive night shifts, are difficult to reconcile with published data on accident risk. In light of this, we have examined two large sets of alertness ratings with a view to further refining the model and identifying additional factors that may influence alertness at any given point in time. Our results indicate that, at least for the range of sleep durations and wake-up times commonly found on rotating shift systems, we may assume the phase of the endogenous circadian component of alertness (process C) to be "set" by the time of waking. Such an assumption considerably enhanced the predictive power of the model and yielded remarkably similar phase estimates to those obtained by maximizing the post-hoc fit of the model. We then examined the manner in which obtained ratings differed from predicted values over a complete 8-day cycle of two, 12-h shift systems. This revealed a pronounced "first night compensation effect" that resulted in shift workers rating themselves as progressively more alert than would be predicted over the course of the first night shift. However, this appeared to be achieved only at the cost of lowered ratings on the second night shift. Finally, we were able to identify a "time on shift" effect whereby, with the exception of the first night shift, alertness ratings decreased over the course of each shift before showing a modest "end effect." We conclude that the identification of these additional components offers the possibility that in the future we may be able to predict trends in accident risk on abnormal sleep-wake schedules.

The Correlation Between Activity, Blood Pressure and Heart Rate in Freely Mobile Sprague-Dawley and Transgenic Rats

Systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR) and locomotor activity have been measured at 1-min intervals for 24 h in Sprague-Dawley (N = 5) and for 2x24 h in transgenic hypertensive (N = 4) rats. The animals were freely mobile and entrained to a 12:12 LD cycle (lights on at 0700). The endogenous circadian component of the cardiovascular variables was removed from the raw data, and then correlations between activity and the residual component (raw data minus the endogenous component) of SBP, DBP and HR were calculated. This calculation was performed twice, in the mid-light and mid-dark phases. We have investigated if the mean size of the correlation coefficients depended on cardiovascular variable (SBP, DBP or HR), phase (D or L) or strain (Sprague-Dawley, SPD, or Transgenic, TG, rats). Nearly all correlations were positive and ANOVA's showed a significant effect of cardiovascular variable for both strains, with correlations for HR being significantly higher than those for SBP and DBP. The mean correlations in the SPD strain were significantly higher than in the TG strain for variables SBP and DBP, but not for HR. The correlations between activity and blood pressure were more marked for SPD rats in the light (inactive) than dark (active) phase. Both strains showed ultradian rhythms in all variables, particularly in the light phase. If the analysis was repeated using deviations of the cardiovascular variables from a 1-h moving average rather than the endogenous circadian component, then the results were very similar. The results are discussed in terms of the links between the rhythms of activity and cardiovascular variables, with particular reference to differences between the two strains.

Melatonin and the Circadian Regulation of Sleep Initiation, Consolidation, Structure, and the Sleep EEG

The endogenous circadian rhythm of melatonin, driven by the suprachiasmatic nucleus, exhibits a close association with the endogenous circadian component of the sleep propensity rhythm and the endogenous circadian component of the variation in electroencephalogram (EEG) oscillations such as sleep spindles and slow waves. This association is maintained even when the sleep-wake cycle is desynchronized from the endogenous circadian rhythm of melatonin. Administration of melatonin during the day increases daytime sleep propensity as indexed by both the latency to sleep onset and sleep consolidation. The EEG during daytime sleep after melatonin administration exhibits characteristics reminiscent of the nocturnal sleep EEG, that is, increased sleep spindle activity and reduced slow-wave sleep and slow-wave activity, as detected by quantitative EEG analysis. Administration of higher doses of melatonin (5 mg or more) prior to nocturnal sleep results in an increase in rapid eye movement (REM) sleep. These data demonstrate that melatonin exerts effects on the main characteristics of human sleep, that is, latency to sleep onset, sleep consolidation, slow waves, sleep spindles, and REM sleep. There is a need for further studies using physiological doses and delivery systems that generate physiological plasma melatonin profiles to firmly establish the role of the endogenous circadian rhythm of melatonin in the circadian regulation of sleep.

Involvement of circadian rhythm in response to ultrashort photoperiods in blackheaded bunting, Emberiza melanocephala

Groups of photosensitive adult female blackheaded buntings were exposed to various ultrashort days photoperiodic regimes for 60 days in which a fixed 3 h photophase was coupled with dark phases in cycles of 22 to 32 hours duration. One group of buntings was kept in long days (15L:9D) as control. Significant increase in ovarian weight and circulating plasma estradiol concentration was marked in the cycles of 30 h (3L:27D), and 32 h (3L:29D) photoperiodic schedules as well as in control group (15L:9D), whereas there was no response in the cycles of 22 h (3L:19D) and 24 h (3L:21D). It seems that the response to ultrashort day cycles is due to a phase advance or delay in photosensitivity of the response system repeatedly shows coincidence of the external photophase (3 h) with the photoinducible phase of an endogenous circadian rhythm. Therefore, the present result appears to be consistent with Bünning hypothesis suggesting the involvement of an endogenous circadian component in the female blackheaded bunting.

Internal rhythms in humans

Abstract Human physiology and behavior are characterized by a daily internal temporal dimension. This so-called circadian rhythmicity is present for almost all variables studied to date, persists in the absence of external cycles, and is synchronized to the external 24-h world by an internally generated circadian rhythm of light sensitivity. The light-sensitive circadian pacemaker, presumably also in humans located in the suprachiasmatic nucleus of the hypothalamus, drives the endogenous circadian component of rhythmicity for a number of variables including plasma melatonin, alertness, sleep propensity and sleep structure. Overt rhythmicity and the consolidation of vigilance states are generated by a fine-tuned interaction of this circadian process with other regulatory processes such as sleep homeostasis.

Circadian variation of ambulatory myocardial ischemia. Triggering by daily activities and evidence for an endogenous circadian component.

The morning peak in myocardial ischemia has been related to diurnal variations in physical and mental activities and to postural changes upon awakening. This study assesses (1) the effects of exogenous activity triggers at different times of the day and (2) the contribution of an endogenous (ie, activity- and posture-independent) circadian vulnerability for ambulatory ischemia. Sixty-three stable coronary artery disease patients underwent ambulatory ECG monitoring and completed a structured diary assessing physical and mental activities. During 2519 hours of observation, a morning increase in ischemia coincided with increases in physical and mental activities, and an evening decrease in ischemia coincided with a decline in activities. During the morning, ischemic versus ischemia-free periods were more likely to occur with high levels of physical activity (P < .001). High physical activity triggered ischemia to a lesser but still significant extent (P < .05) in the afternoon but not in the evening (P = NS). High levels of mental activity triggered ischemia significantly during the morning (P < .04) and evening (P < .04) but not in the afternoon. When a residualized score procedure was used to correct ischemic time for each patient's simultaneously measured activities, for hourly heart rates, or for activity-related heart rate fluctuations, the circadian variation in ischemia was still observed (P < .001), with a peak at 6 AM. A significant increase in ischemia occurred immediately after awakening (P < .05), but activity-adjusted increases in morning ischemia persisted (P < .05) for 2 hours after awakening. Exogenous factors (physical and mental activities) are most potent as triggers of ischemia during the morning hours, and the postural change after awakening contributes to the morning increase in ischemia. There is also evidence for an endogenous, activity-independent circadian influence on ischemic susceptibility that is independent of exogenous factors and that sustains the increase in ischemia upon awakening.

Circadian rhythm of heat production, heart rate, and skin and core temperature under unmasking conditions in men

Seven healthy men were studied in a 34-h constant routine protocol to investigate whether the daily rhythm of heat production and heat loss has an endogenous circadian component. Under these unmasking conditions (constant bed rest, no sleep allowed, regular food and fluid intake), a significant circadian rhythm could be demonstrated for heat production, heart rate, and skin temperatures but not for the respiratory quotient. Heat production and heart rate were phase locked with a maximum at 1100-1200 h. Proximal skin temperatures (infraclavicular region, thigh, and forehead) followed the same circadian rhythm as rectal temperature, whereas distal skin temperatures (hands and feet) were opposite in phase. These physiological circadian rhythm parameters, as well as biochemical parameters (urinary sodium, potassium, urea, and urine flow), were phase advanced by 25-180 min with respect to the circadian rhythm in rectal temperature. Our findings under unmasking conditions show that the circadian variation in rectal temperature is a consequence of endogenous circadian rhythms in both heat production and heat loss.

Burrow plugging in the crab Uca uruguayensis and its synchronization with photoperiod and tides

Field observations and laboratory experiments were performed to analyze the burrow plugging behavior of U. uruguayensis and to analyze its relation to two environmental cycles: light-dark cycle and tides. Field observations showed that burrow plugging is a rhythmic behavior synchronized with both environmental cycles such that burrows are open during those periods of simultaneous light and low tide. Laboratory experiments suggested that the plugging rhythm is under endogenous circadian control, whereas its synchronization with the tidal cycle, particularly with periodic inundation, seems to be strongly exogenous, not showing clear circatidal components. It is proposed that burrow plugging is adaptive because it allows the animals to be within an air medium, more suitable for their respiration modality, during high tide and because it prevents burrow collapse. It is also proposed that both the endogenous circadian component and the lack of an endogenous circatidal component can also be explained on the basis of adaptive value, taking into account the regular temporal structure of the solar day and the irregular temporal structure of the tidal cycle.

Biological clock in seasonal reproductive cycle in the ditch shrimp Palaemonetes varians Leach. I. Photoperiodic time measurement

Two experiments were performed to examine responses of the ditch shrimp Palaemonetes varians Leach to various photoperiodic cycles. In a first series of experiments, stimulations of reproduction were analyzed by recording responses of the females to six photophases from 6 to 18 h length, and continuous darkness or light. The second experiment applied a series of symmetrical and asymmetrical “skeleton” photoperiods. The photoperiodic response curve showed that long days stimulate reproduction. The features of this curve revealed the existence of a biological clock capable of time-measurement. Results of a “resonance” experiment provided evidence of the involvement of an endogenous circadian component in the photoperiodic clock. The internal coincidence model developed for insects proved adequate to describe the results of skeleton photoperiod experiments. Relations between reproduction and moulting were then examined. Among Crustacea, results were compared with the data obtained in the terrestrial isopod Armadillidium vulgare.

Influence of Melatonin on the Sleep‐Independent Component of Prolactin Secretion

Sleep has a stimulatory effect on prolactin secretion. Recent studies in the human suggest the hypothesis that prolactin has also an endogenous sleep-independent rhythm, which can be influenced by endogenous melatonin. To investigate this hypothesis, the prolactin response to nighttime exposure to bright light was studied in eight women in detail. Light exposure induces a decrease in nocturnal melatonin secretion. It was demonstrated that exposure to bright light for 2 h at night caused a decrease in prolactin secretion, which surpassed significantly the decline one would expect by sleep deprivation only (P less than 0.01). This was associated with a similar decline in melatonin secretion. Fall and rise of prolactin secretion under these conditions were always preceded by decrease and rise in melatonin levels in all eight women studied. Based on these observations, it is concluded that melatonin is associated with an endogenous circadian component of prolactin secretion. As specific melatonin receptors have been identified in the human nucleus suprachiasmaticus, which is the "master" circadian pacemaker, the observed phenomenon might be mediated through this structure. An alternative explanation of our findings could be based on the fact that melatonin influences dopamine metabolism, which in turn alters prolactin secretion. It can also not be ruled out that melatonin might act via the opioid system, which then could affect prolactin secretion. The total secretory activity for both hormones (area under the curve) did not change under experimental conditions, when compared to a control group. This suggests that acute light exposure and sleep deprivation influence the secretory process rather than the synthesis of these two hormones. This is in agreement with the observation that changes in natural light exposure throughout the year do alter the amplitude, but not the total amount of melatonin secreted. Further studies are needed to answer the question of melatonin storage definitively, as it is commonly believed that melatonin is immediately released after synthesis. It is concluded that melatonin through its external modulator light might entrain the circadian sleep-independent component of prolactin secretion and via its action on prolactin could modulate reproductive processes.

A comparison of the activity patterns of the sand goby Pomatoschistus minutus (Pallas) from areas of different tidal range

Sand gobies (Pomatoschistus minutus) collected from beaches with a large tidal range (Scotland) exhibit a circatidal rhythm of activity in constant conditions in the laboratory. There is no endogenous circadian component to the rhythm. The phasing of the rhythm is such that peak activity occurs at the predicted time of ebb tide. Light‐dark (LD) cycles applied in the laboratory have a marked effect on activity greatly enhancing it at night so that the original tidal rhythm becomes nocturnal. Some evidence was obtained that LD cycles can entrain a weak nocturnal circadian rhythm in fish removed from tidal conditions. Contrary to expectations, some fish from beaches with a small and unpredictable tidal range (Oslofjord, Norway) also show a weakly persistent circatidal thythm without an endogenous circadian component, but great variability was noticed between individual fish. Laboratory LD cycles did not entrain a persistent circadian rhythm in the fish from the Oslofjord.

Diel activity of the pink shrimp Penaeus duorarum

Twelve juvenile pink shrimp, Penaeus duorarum Burkenroad, were tested individually for 3-day periods in electronic shuttleboxes to determine their diel patterns of locomotor activity, in relation to a natural summer photoperiod. Nocturnal activity was twice that exhibited during the daytime; however, a bimodal pattern was evident with crepuscular peaks occurring at dawn and dusk. The dusk peak was more pronounced, with activity increasing markedly before sunset (indicative of an endogenous circadian rhythm component), and continuing during the initial hours of darkness, gradually declining later during the night with a secondary peak at dawn falling off to minimal activity during daylight. Crepuscular activity (mean of dawn and dusk) was twice the nocturnal average.

The photoperiodic clock in the flesh-fly, Sarcophaga argyrostoma

Larval cultures of the flesh-fly, Sarcophaga argyrostoma, were raised in experimental light cycles with periods ( T) of 21 to 72 hr, each cycle containing a photoperiod of 4 to 20 hr of white light. This ‘resonance’ technique revealed periodic maxima (∼24 hr apart) of pupal diapause, thereby demonstrating an endogenous circadian component in the photoperiodic clock. The positions of these maxima of pupal diapause suggested that the oscillation, like that controlling the pupal eclosion rhythm in Drosophila pseudoobscura, is ‘damped out’ by photoperiods longer than about 11 to 12 hr, but restarts at dusk whereupon it runs with circadian periodicity in a protracted dark period. With photoperiods shorter than 12 hr, however, the two diapause maxima were less than 24 hr apart, suggesting that an additional component, possibly a ‘dawn hour-glass’, was modifying the position of the first peak. Both photoperiod and the period of the driving light cycle ( T) were shown to affect the length of larval development (the sensitive period) and the number of calendar days needed to raise the incidence of pupal diapause to 50 per cent (the required day number, RDN). Peaks of diapause induction were shown to be the result of an interaction between a long sensitive period (slow development) and a low RDN, whereas troughs in diapause induction were the result of an interaction between a short sensitive period (fast development) and a higher RDN. Larvae of S. argyrostoma are unable to distinguish (in a photoperiodic sense) between 12 and 18 hr of red light (600 nm).