Onset Of Daily Activity Research Articles

Daily Behavioral Rhythmicity and Organization of the Suprachiasmatic Nuclei in the Diurnal Rodent, Lemniscomys barbarus

Wheel‐running activity was recorded in Lemniscomys barbarus exposed to different lighting conditions. This rodent shows rhythmic locomotor activity under natural twilight‐light/dark (LD) as well as squared‐LD cycles. A mean of 77% of the activity occurred during the light phase. Under different controlled photoperiods, the quantity of daily locomotor activity was relatively stable except for a lower level in the shortest photoperiod tested (LD 06∶18). The duration of the active phase tended to increase with the duration of the light phase, especially in the longer photoperiods. Whatever the lighting conditions, Lemniscomys barbarus started running before lights‐on and stopped after lights‐off. The phase angle of activity offset relative to lights‐off was stable in each squared‐photoperiod, whereas the phase angle of activity onset relative to lights‐on was significantly the highest under the shortest photoperiods. Recording of activity under constant lighting conditions showed that the daily rhythm of locomotor activity is fundamentally circadian. The endogenous period was slightly<24 h (mean=23.8 h) in permanent darkness and>24 h (mean=24.5 h) in continuous light. Re‐entrainment of the locomotor activity rhythm after a 6 h phase advance or delay requires only four days on average. Moreover, the phase‐responses curve to a 30 min light pulse (200 lux) in Lemniscomys barbarus kept in constant dark reveals large phase shifts according to circadian times (CT). With CT0 being defined as the onset of daily activity, maximum phase delay and advance shifts were observed at CT11 (Δ Ψ=‐5.7 h±2.3 h) and CT21 (Δ Ψ =4.9±1.2 h), respectively. Interestingly, the phase‐response curve to light did not show any dead zone. Immunohistochemical staining of the suprachiasmatic nuclei indicates that arginine vasopressin‐immunoreactive cell bodies and fibers delimited a dorsal subregion that extends laterally and medially. The ventral subregion is rich in vasoactive intestinal peptide‐immunoreactive neurones overlapping a smaller area containing gastrin‐releasing peptide‐expressing cells and receives numerous fibers labeled with neuropeptide Y antibody. The results of this study clearly demonstrate that Lemniscomys barbarus is a diurnal species highly sensitive to the shifting effects of light. Overall, this rodent can be considered a new and interesting model for circadian rhythm neurobiology.

The enigma of behavioral inputs to the circadian clock: A test of function using restraint

Wheel running stimulated during the daily rest period can acutely shift circadian rhythms in Syrian hamsters. Spontaneous running, defining the active phase of the circadian rest–activity cycle, can shorten the circadian periodicity in constant light or dark in several nocturnal rodent species. The adaptive significance of these behavioral effects on pacemaker phase and period is unclear. Here we consider a hypothesis that behavioral inputs to the circadian pacemaker serve primarily to enhance the precision of light–dark entrainment and maintain daily activity onset close to lights-off (i.e., dusk) by stabilizing entrainment on a steeper portion of the delay zone of the phase–response curve to light. This hypothesis rests on the evidence that spontaneous activity early in the active period feeds back on the pacemaker to advance its motion. If so, then preventing activity at this time should induce a phase delay shift. Such delay shifts have been reported in Syrian hamsters physically restrained early in the active period. We show here that restraint can induce phase delays but that, using the Aschoff Type 2 procedure for measuring shifts, these delays are very small, are inversely related to behavioral sleep during restraint, and are positively correlated with ‘rebound’ increases in running following restraint, at a circadian time when stimulated running is known to induce phase delay shifts. Repeated bouts of restraint, to promote habituation, were associated with strong attenuation of ‘rebound’ running and no significant delay shifts. These results suggest that, in Syrian hamsters, spontaneous activity early at night has little effect on pacemaker motion, and argue against the stated hypothesis.

Olfactory cues accelerate reentrainment following phase shifts and entrain free-running rhythms in female Octodon degus (Rodentia).

Social interactions between conspecifics is a type of nonphotic zeitgeber common to several species. In the diurnal rodent Octodon degus, social interactions enhance reentrainment after phase shifts and can act as a weak zeitgeber. Olfactory stimuli appear necessary for these effects since bulbectomy eliminates socially enhanced reentrainment. In Experiment 1, the authors examined whether stimulation of the main olfactory system was sufficient to enhance reentrainment after 6-h phase advances and delays in the adult female O. degus. When test animals received conspecific odor cues during reentrainment, they entrained 39% faster after phase advances (p < 0.05) and 33% faster after phase delays (p < 0.001) than when they did not receive odor cues. Thus, olfactory cues from distant female donors were sufficient to enhance rates of entrainment in female O. degus and provided results equivalent to earlier studies with donors and shifters housed in the cages together. In Experiment 2, the authors examined whether discrete 3-h and 1-h daily pulses of airborne odors from a group of 5 entrained female degus would be sufficient to produce entrainment of wheel-running activity in adult female conspecifics. During the period of exposure to 3-h pulses, 50% (4/8) of the subjects temporarily entrained to a 24-h cycle, while 12.5% (1/8) of the subjects fully entrained. Exposure to 1-h pulses allowed 37.5% (3/8) of the subjects to temporarily entrain and 12.5% (1/8) of the subjects to fully entrain. Duration of entrained episodes was positively correlated with psi, daily onset of activity with respect to the timing of odor exposure (Pearson r = 0.731; p < 0.05), such that animals with the entraining odor pulse beginning during subjective day (psi = 7.8 h, CT 7.8 +/- 1.4) had longer periods of entrainment (22.2 +/- 5.6 days) than animals with the entraining pulse occurring during subjective night (psi = -4.6 h; CT 19.4 +/- 0.9; 5.6 +/- 0.9 days; p < 0.001). In addition, for each animal, the combined duration of all episodes of 24-h entrainment correlated with increased period length (tau) of free-running rhythms (Pearson r = 0.733; p < 0.05). Thus, daily discrete pulses of odors with durations of either 1 or 3 h from female conspecifics were sufficient to produce both temporary and full entrainment to a 24-h cycle in the majority of female O. degus, and the likelihood of long periods of entrainment correlated with long taus and coordination of the odor pulse with mid subjective day.

Rhythm chronotypes in a diurnal rodent, Octodon degus.

Interindividual variations in entrained human circadian rhythms and their relationship to differences in ease of adjustment to phase shifts have been well described. Humans classified as morning type (MT) display phase-advanced temperature minimum, onset of daily activity, and onset of sleep, whereas evening type (ET) individuals demonstrate the opposite pattern with a later phase angle of entrainment for these measures. We examined 15 variables of circadian activity and temperature for diurnal Octodon degus (n = 49; 28 male, 21 female), and noted similar chronotype variations as described for humans. Individuals with extreme MT and ET chronotypes were identified as those who differed from the entire population mean by one or more standard deviations. Compared with MTs (n = 5) and intermediate chronotypes (ITs, n = 34), ETs (n = 10) displayed a significant delay in the phase of temperature minimum, rise in temperature above the daily mean, and decline in activity and temperature below the daily mean, and had longer elevated activity and temperature durations and higher levels of mean daily activity and activity amplitude. MTs also displayed a significant phase advance in the morning temperature rise compared with ITs and ETs. This is the first description of variation in entrained circadian rhythms similar to that described for humans in both an outbred animal population and a diurnal rodent.

Melatonin and the Circadian Clock in Mink: Effects of Daily Injections of Melatonin on Circadian Rhythm of Locomotor Activity and Autoradiographic Localization of Melatonin Binding Sites

The present study examines a putative effect of exogenous melatonin on the circadian organization of the mink. Two approaches were used to determine first whether entrainment of free-running rhythms of locomotor activity in constant darkness can be obtained by daily melatonin injections, thus demonstrating a control of melatonin on the clock generating circadian rhythms, the suprachiasmatic nucleus of the hypothalamus. Entrainment was never obtained in the 8 vehicle-injected females and 7 out of the 8 melatonin injected-ones. In 3 females free-running in constant darkness, a phase advance followed by a few days of transient effect was observed when melatonin injections coincided with the onset of activity. However, the comparison of the regression of the daily activity onset related to successive days by covariance analysis revealed that true entrainment was effective in only 1 female. Second, we examined the distribution of melatonin binding sites within the brain of juvenile and adult mink using an in vitro autoradiographic procedure with [125I]2-iodomelatonin. No binding sites were observed in the suprachiasmatic nucleus of any of the animals. However, all animals displayed a high density of melatonin binding sites in the pars tuberalis of the pituitary. The relation between a modulatory control of melatonin on the circadian clock and the presence and density of melatonin binding sites in the clock is discussed. In mink, melatonin does not seem to act as an internal Zeitgeber.

A nonphotic stimulus causes instantaneous phase advances of the light- entrainable circadian oscillator of the Syrian hamster but does not induce the expression of c-fos in the suprachiasmatic nuclei

The study investigated whether nonphotic cues that alter the phase of overt circadian rhythms do so by causing instantaneous shifts in the underlying, light-sensitive clock. Wheel-running activity in Syrian hamsters was studied under free-running conditions of constant dim red light as an overt marker of circadian phase, the daily onset of activity being defined as circadian time 12 (CT 12). Exposure to a 15 min pulse of bright light at CT 12.20 caused a phase delay in activity onset, whereas pulses delivered at CT 11.20 had no effect upon the overt rhythm. Correlated with their effect on behavior, light pulses delivered at CT 12.20 induced expression of c-fos-like immunoreactivity in the retinorecipient regions of the suprachiasmatic nuclei of the hypothalamus (SCN), whereas pulses delivered at CT 11.20 had no effect upon the expression of c-fos. Expression of this immediate-early gene therefore provided a second marker of circadian phase, because its induction by light is closely correlated with the onset of subjective night (CT 12). To establish a suitable protocol for nonphotic shifts of the activity rhythm, animals were handled and received a subcutaneous injection of saline at different circadian phases. Injections at CT 8 or CT10 caused an immediate bout of wheel-running activity, and a consequent phase advance in the activity rhythm as assessed by the earlier onsets of activity in successive days. Handling and injections at other circadian phases were without effect. Despite shifting the overt rhythm, these procedures at CT 10 did not lead to the expression of c-fos in the SCN.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of stress on adrenocortical function in the male tree shrew ( Tupaia belangeri)

Adrenocortical function in the tree shrew ( Tupaia belangeri) was evaluated by reference to levels of individual corticosteroids as an initial phase in the clarification of the hormonal mechanisms by which stress influences male reproductive function. Corticosteroids were separated with Sephadex LH-20 chromatography and measured by specific radioimmunoassay procedures. Corticosterone was the principal corticosteroid in the peripheral plasma and in unstressed animals the ratio corticosterone: cortisol was 4.5:1 (1000 hr samples). Basal levels of corticosterone and cortisol exhibited parallel diurnal fluctuations with a peak at 0600 hr preceding the onset of daily activity and similar to that described for other diurnally active species. Levels of corticosterone and cortisol were markedly elevated 10 min after either restraint stress applied at various times during the day or ACTH administration to both saline- and dexamethasone-pretreated animals. In each of these experimental situations cortisol showed a greater relative increase from basal levels as compared to corticosterone resulting in a decline in the ratio corticosterone:cortisol. For example, the unstressed ratio of corticosterone (9.1 ± 0.8 ng/ml) and cortisol (2.0 ± 0.2 ng/ml) at 1000 hr was 4.5:1. Following 10 min of restraint stress the ratio declined to 1.5:1 (corticosterone, 44.9 ± 1.4 ng/ml; cortisol, 31.7 ± 1.3 ng/ml). An increase in cortisol is considered to be an important component of the physiological response to stress by the tree shrew adrenal. It is concluded that the adrenal cortex of the tree shrew normally produces corticosteroids through two pathways and the secretion of both corticosterone and cortisol are responsive to acute ACTH stimulation and feedback inhibition by dexamethasone. The lack of dissociation in the response of these two steroids following acute stress, dexamethasone supression, and ACTH stimulation indicates that their secretion in controlled through a common mechanism in accordance with the general principles established by reference to individual corticosteroids in more conventional experimental animals.

Neuropeptide Y: Role in light-dark cycle entrainment of hamster circadian rhythms

Microinjection of neuropeptide Y (NPY) into the suprachiasmatic region of the hypothalamus (SCN) phase shifted the circadian activity rhythm of hamsters housed in constant light. NPY advanced the phase when injected during the 12 h that preceded the daily onset of activity and tended to phase delay the activity cycle when injected during the 12 h after activity onset. In contrast, injection of saline into the SCN or NPY into the ventricular system had no effect on circadian phase. These and other data suggest that NPY functions as a chemical messenger important for the light-dark cycle entrainment of circadian rhythms.

Activity patterns of the Red squirrel (Sciurus vulgaris)

AbstractDaily and seasonal patterns of free‐ranging Red squirrels (Sciurus vulgaris) were studied by means of radiotracking. On a seasonal basis, two distinct daily activity rhythms were exhibited. In winter, the squirrels had a short, uninterrupted active phase which is probably an adaptation for conserving energy by minimizing the period of heat loss. In summer, the active phase was more than twice as long as in winter, and was usually broken by a rest period. The activity patterns at the equinoxes were intermediate between summer and winter patterns.In summer, the biphasic pattern of daily activity of individuals gave rise to a trend towards bimodality in the activity pattern of the population as a whole, although the rest periods and second active periods of different squirrels were not synchronized. Onset of daily activity was associated with daybreak, but cessation occurred well before nightfall.Superimposed onto the basic seasonal patterns of activity were the effects of feeding requirements, foraging accounting for a mean of 82% of a month's activity. As well as immediate nutritional requirements, the habit of scatter‐hoarding mast had a marked influence on the duration of foraging, causing a peak in activity in September. Although throughout most of the year the foraging time appeared to influence the duration of activity, active time in late winter appeared to be self‐determining, causing squirrels to lose body‐weight rather than to extend the foraging period. Other factors affecting activity patterns are discussed.

Effect of one-second light pulses on testicular function and locomotor activity in the golden hamster.

The effects of 1-sec light pulses on testicular size and the circadian rhythm of locomotor activity in male golden hamsters were examined utilizing two different paradigms. First, adult male hamsters that had been maintained on 14L:1OD were transferred to either a 6L:18D or 6L:18D light cycle with a 1-sec light pulse interrupting the dark phase of 8 h after lights-off. Exposure to 6L:18D for 11 weeks induced complete testicular regression which was accompanied by low serum testosterone levels. In contrast, the paired testis weights and serum testosterone levels of animals receiving the 1-sec night interruption were maintained at values comparable to those found in hamsters maintained on photostimulatory long days. The daily onset of activity occurred 4-6 h after lights-off in the animals exposed to 6L:18D. Animals receiving the 1-sec night interruption exhibited two distinct patterns of entrainment: one in which the onset of activity occurred near the time of the 1-sec light pulse, and another in which activity was initiated daily near the offset of the 6-h light phase. In the second study, adult male hamsters that had been maintained on 14L:1OD were transferred to either constant darkness or to a 1-sec skeleton photoperiod consisting of two 1-sec light pulses separated by dark intervals of 14 h and 10 h (i.e., 1 sec L:14D:1 sec L:IOD). As expected, exposure to constant darkness for 11 weeks induced complete testicular regression. However, in 7 of 12 animals exposed to 2 sec of light per day, testicular size was maintained in the range of values normally observed in hamsters exposed to stimulator>’ long-day photoperiods. Following transfer to constant darkness, the circadian rhythm of locomotor activity was observed to free-run with a period greater than 24 h in all animals. In contrast, entrainment of the activity rhythm was observed in all animals exposed to the 1-sec skeleton photoperiod with the pattern of activity resembling that exhibited by animals maintained under a complete 14L:1OD photoperiod. These results demonstrate that as little as 2 sec of light per day can influence the neuroendocrine events underlying the photoperiodic maintenance of gonadal function, and can be effective stimuli for the entrainment of the circadian rhythm of locomotor activity.

Distribution of some daily and seasonal events in relation to changes of physical factors.

A special case of the Weibull distribution model is used in describing the course of behavioural transformation processes in relation to some cyclic physical factor. The model assumes that the rate of the process increases, the less inhibiting the physical factor, and the faster the factor changes. However, due to some resistance or a depletion, the rate slows down, the further the process progresses. The model was tested on the daily onset of activity in nocturnal insects, daily roosting flight of blackbirds, dark and light adaptation by pigment migration in insect eyes, photoperiodic response of an insect, and daily emergence of tiger beetles. The assumptions of the model are tested and discussed. One of these is violated in unnaturally fast changes of the physical factor because the process reaches some constant minimum duration, and proportionality between rate of process and rate of factor can no longer be maintained.

Pineal melatonin synthesis in Syrian hamsters: a summary.

During the past decade there has been ample documentation of the proposition that the pineal gland mediates photoperiodic influences upon reproductive behavior of hamsters. It is commonly hypothesized that the pineal gland expresses its activity by transformation of photoperiodic information into an hormonal output, that hormone being melatonin. If this hypothesis is correct, there must be some essential diffrence in melatonin's output when hamsters are exposed to different photoperiodic environments. The experiments summarized in this communication analyze pineal melatonin contents in Syrian hamsters maintained in a variety of photoperiodic conditions during different physiological states. The results demonstrate that adult hamsters have a daily surge in pineal melatonin content throughout their lifetime when exposed to simulated annual photoperiodic cycles. There is some fluctuation in the amount of pineal melatonin produced during different physiological states and photoperiodic environments, but these fluctuations seem small when compared to those normally found for other regulatory hormones. When hamsters are exposed to different photoperiodic regimens, the daily melatonin surge maintains a relatively constant phase relationship with respect to the onset of daily activity. There is a concomitant change in its phase relationship with respect to light-dark transitions.

Effect of progesterone on the estrous activity cycle of the rat

Two, 3 or 4 Silastic capsules of progesterone were implanted sc for 24–35 days, removed for a 12–14 day interval and re-implanted into the same adult female rats from which they were taken. A control group received cholesterol capsules. For approximately 4–10 days, Silastic capsules of progesterone suppressed estrous activity cycles. Thereafter, amplitude of running increased progressively and definite periodicities re-emerged. Following a 12 day recovery period, the same capsules were re-implanted and activity was again suppressed. During the first implantation, the number of estrous cycles with greater than 4-day periods significantly increased. Progesterone was also found to delay the daily onset of activity in relation to the light/dark cycle (phase angle difference). Phase angle difference prior to the first implantation was negatively correlated with the rate of recovery. The data support the conclusion that progesterone affects parameters of estrous activity by antagonizing the actions of estrogen and suggest that animals may make internal adjustments to high titers of progesterone.

Center‐of‐gravity of circadian activity and its relation to free‐running period in two rodent species

Abstract A comparative analysis is presented which validates the practice of measuring free‐running period (r) of circadian locomotor activity by means of the slope of a line eye‐fitted to daily onset of activity. The slope of a linear regression calculated through daily onset of activity gives virtually the same ras the slope of the eye‐fitted line. The center‐of‐gravity (COG) of locomotor activity (defined as the mean daily clock time of all recorded activity impulses) is uniquely defined and free of subjective evaluation. COG is shown to be a meaningful representation of the central tendency of circadian activity; calculation of a regression through daily COG of activity also provides an accurate measurement of #GT.

Changes in locomotor activity associated with the photoperiodic response of the testes in male golden hamsters

1. Male golden hamsters were exposed to either 14 h of light per 24 h (LD 14∶10) or LD 6 ∶18 for 210 days, in order to assess the effects of the light-dark cycle and changes in the reproductive system upon locomotor activity. Wheel-running activity was recorded continuously, and testicular size and serum testosterone concentrations were measured periodically throughout the study. 2. The short photoperiod induced a decrease in testicular width, serum levels of testosterone, and the number of wheel revolutions per 24-h period, when compared to the levels of these variables exhibited by LD 14∶10 hamsters. Prolonged exposure to LD 6∶18 resulted in a spontaneous increase in these three parameters to levels indistinguishable from those observed in LD 14∶10 animals. Neither testicular width, serum testosterone concentration, nor quantity of activity changed significantly in LD 14∶10 hamsters throughout the study. 3. LD 6∶18 exposure also induced an expansion of the daily activity time; this increased duration of the daily active phase persisted, even after the spontaneous increase in testicular width, serum testosterone concentration, and number of wheel revolutions per day. The duration of the daily active phase was unchanged in LD 14∶10 animals through the course of the experiment. 4. The onset of activity in all hamsters exposed to LD 14∶10 occured between 0–1 h after lights-off and did not vary appreciably for individual animals during the 210 days of LD 14∶10. The time required for stable reentrainment following a shift from LD 14∶10 to LD 6∶18 varied between 30 and 120 days among individual hamsters, and the phase relationship of activity onset to lights-off after 200 days of LD 6∶18 ranged between -1 and — 6 h. The time course of reentrainment to short days did not appear to be affected by changing testosterone levels. 5. Coincident with LD 6∶18-induced testicular regression was an increased lability of the time of activity onset each day. A return to stability in the day-to-day time of activity onset was coincident with spontaneous testicular recrudescence in LD 6∶18 hamsters. Hamsters maintained on LD 14∶10 exhibited a stable time of daily activity onset throughout the investigation. 6. These results indicate that the number of wheel revolutions per day and the lability of daily activity onset are correlated with light-induced changes in the hamster reproductive system. In contrast, the duration of the daily active phase and the phase relationship between activity onset and lights-off are relatively independent of changes in the reproductive system and are a function of the entraining light-dark cycle.

Latitudinal and population specific differences in timing of daily and seasonal functions in redpolls (Acanthis flammea).

1. The daily (circadian) rhythm of activity and rest of common redpolls (Acanthis f. flammea L.) from 65°N lat. was measured for about one year in individuals outdoors at two latitudes (48° and 65°N). During winter at both latitudes, activity-time (α) of common redpolls approximated the duration of daylight (including civil twilights); onset of activity, however, occurred at lower light intensities than end of activity. During mid-summer, a rest-time (p) of ca. 5 h was maintained at both latitudes. During the times of spring and fall migration (extending into summer or winter, respectively), common redpolls showed nightly unrest or shifted their onset of daily activity into the pre-dawn hours. 2. Redpolls of two subspecies (A. f. flammea and A. f. cabaret Müller) resident at different latitudes (ca. 65° and 49°N) were maintained and measured under the same light conditions at 48°N lat. The two populations showed significant differences in their responses to the same annual changes in day length which included: (i) differences in the timing of the circadian activity rhythm with respect to the daily solar cycle; (ii) differences in the amount and range of seasonal changes in nightly unrest; (iii) differences in the mean level and range of seasonal changes of body weight; and (iv) differences in the timing of postnuptial molt. 3. In all experimental groups of redpolls, the highest "precision" (i.e. lowest mean day-to-day variation) in onset and end of activity was observed when both phases occurred during the twilights. The different effects of annually changing light conditions on onset and end of activity with respect to solar time indicated that timing of these two phases of the activity rhythm is independently controlled. It is further indicated that an annually changing sensitivity to light controls the termination of activity in common redpolls, although ambient temperature can modify this response. 4. Besides the direct influence of latitude on timing and the amount of changes of various circadian and annual functions throughout the year, the important role of long-term (genetic?) adaptation to the light and temperature conditions prevailing in the respective habitats of redpoll populations is emphasized by the results of this study.

Environmental control of the daily onset of luminescent activity in glowworms and fireflies (Coleoptera: Lampyridae).

The daily onset of activity in fireflies and glowworms is a complicated process involving both a circadian rhythm and a triggering mechanism controlled by ambient illumination. Onset of the luminescent activity has been investigated in the field and under semi-natural and experimental conditions. 1. the onset of flashing or glowing occurs at a critical low illumination during or just after the twilight period. This was determined in the adult glowworm Lampyris noctiluca in Denmark and in the fireflies Photuris congener and Photinus umbratus in Florida. 2. The dispersion of activity onsets differs in the three species which is partly due to differences in duration of the decrease of illumination (duration of twilight). At the long duration of twilight in Denmark, L. noctiluca has a greater dispersion of onsets than has P. congener at the short twilight in Florida, although the period of activity onsets occurs during about the same range of illuminations. 3. The onset of activity occurs at a lower illumination at low than at high temperatures. 4. At a reduction in illumination to different low values an increasing number of animals does not commence activity above about 1 lux. There is no activity if the illumination remains above 10 lux. 5. A period of latency appears in the onset of luminescent activity by a sudden change in illumination from light to dark (dim light). The duration is about 11 min in L. noctiluca, 5 min in three species of Photuris and 3 1/2 min in P. umbratus. The latency is constant at different illuminations below the threshold for overt activity. The process underlying the latency also occurs at illuminations above the threshold, but it is slowed down the higher the illumination. Further, the latency is shorter at a fast decrease of illumination than at a slow. 6. It is suggested that onset of activity in nocturnal insects involves three steps. A circadian rhythm of sensitization brings the animal into a specific state of receptivity. This happens close to the normal time of activity onset. Further, a latency process controlled by illumination occurs before overt activity can take place. This is then released below a certain threshold level of illumination.