Plasticity of Hamster Circadian Entrainment Patterns Depends on Light Intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (～350 lux) were alternated with either dim (<0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (～45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.     

Dim Nighttime Illumination Interacts With Parametric Effects of Bright Light to Increase the Stability of Circadian Rhythm Bifurcation in Hamsters

The endogenous circadian pacemaker of mammals is synchronized to the environmental day by the ambient cycle of relative light and dark. The present studies assessed the actions of light in a novel circadian entrainment paradigm where activity rhythms are bifurcated following exposure to a 24-h light:dark:light:dark (LDLD) cycle. Bifurcated entrainment under LDLD reflects the temporal dissociation of component oscillators that comprise the circadian system and is facilitated when daily scotophases are dimly lit rather than completely dark. Although bifurcation can be stably maintained in LDLD, it is quickly reversed under constant conditions. Here the authors examine whether dim scotophase illumination acts to maintain bifurcated entrainment under LDLD through potential interactions with the parametric actions of bright light during the two daily photophases. In three experiments, wheel-running rhythms of Syrian hamsters were bifurcated under LDLD with dimly lit scotophases, and after several weeks, dim scotophase illumination was either retained or extinguished. Additionally, “full” and “skeleton” photophases were employed under LDLD cycles with dimly lit or completely dark scotophases to distinguish parametric from nonparametric effects of bright light. Rhythm bifurcation was more stable in full versus skeleton LDLD cycles. Dim light facilitated the maintenance of bifurcated entrainment under full LDLD cycles but did not prevent the loss of rhythm bifurcation in skeleton LDLD cycles. These studies indicate that parametric actions of bright light maintain the bifurcated entrainment state; that dim scotophase illumination increases the stability of the bifurcated state; and that dim light interacts with the parametric effects of bright light to increase the stability of rhythm bifurcation under full LDLD cycles. A further understanding of the novel actions of dim light may lead to new strategies for understanding, preventing, and treating chronobiological disturbances. (Author correspondence: jevans@msm.edu)     

Circadian Rhythm of Acute Phase Proteins under the Influence of Bright/Dim Light during the Daytime

We investigated the influence of two different light intensities, dim (100 lx) and bright (5,000 lx), during the daytime on the circadian rhythms of selected acute phase proteins of C‐reactive protein (CRP), α1‐acid glycoprotein (AGP), α1‐antichymotrypsin (ACT), transfferin (TF), α2‐macroglobulin (α2‐m), haptoglobin (HP), and ceruloplasmin (CP). Serum samples were collected from 7 healthy volunteers at 4 h intervals during two separate single 24 h spans during which they were exposed to the respective light intensity conditions. A circadian rhythm was detected only in ACT concentration in the bright light condition. The concentration of ACT, a positive acute phase protein (APP), increased (significantly significant differences in the ACT concentration were detected at 14:00 and 22:00 h) and AGP showed a tendency to be higher under the daytime bright compared to dim light conditions. There were no significant differences between the time point means under daytime dim and bright light conditions for α2‐M, AGP, Tf, Cp, or Hp. The findings suggest that some, but not all, APP may be influenced by the environmental light intensity.     

Significance of Nonparametric Light Effects in Entrainment of Circadian Rhythms in Owl Monkeys (Aotus Lemurinus Griseimembra) by Light-Dark Cycles

In a total of 12 adult Colombian owl monkeys, Aotus lemurinus griseimembra, the significance of nonparametric light effects for the entrainment of the circadian system by light-dark (LD) cycles was studied by carrying out (a) phase-response experiments testing the phase-shifting effect of 30-min light pulses (LPs) of 250 lx applied at various phases of the free-running circadian activity rhythm (LL 0.2 lx) and (b) synchronization experiments testing the entraining effect of 24-h single LP photoperiods consisting of 30-min L of 80 lx and 23.5-h D of 0.5 lx (sP 0.5) and skeleton photoperiods consisting of two 30-min LPs of 80 lx, given against a background illuminance of 0.5 lx either symmetrically at 12-h intervals (PP 12:12) or asymmetrically at 9- and 15-h intervals (PP 9:15). The phase-response characteristics in Aotus, as evidenced by the phase-response curve, generally correspond to those of nocturnal rodents, proving that this neotropical simian primate chronobiologically is a genuine nocturnal species. When free-running with a spontaneous period close to 24 h (24.3 ± 0.1 h), the PP 12:12 produced entrainment in only two of five owl monkeys, whereas the sP 0.5 entrained four of them. The PP 9:15, however, brought about stable entrainment of the circadian rhythms of locomotor activity, feeding activity, and core temperature in all animals tested (n = 8). Changes in phase position of the activity time with the endogenous rhythm entrained by a PP 12:12, by an sP 0.5, or by a PP 9:15 give evidence that both LPs of a skeleton photoperiod contribute to the phase setting of the circadian system. When free-running with a considerably lengthened spontaneous period (τ ≥ 25.5 h), even the sP 0.5 and the PP 9:15 failed to entrain the owl monkeys' circadian rhythms, whereas a 24-h photoperiod with a very long LP of 3 h caused entrainment. The results indicate that in Aotus lemurinus griseimembra, in addition to the nonparametric light effects, parametric light effects play a significant role in the entrainment of circadian rhythms by LD cycles.     

Circadian rhythms of melatonin in European starlings exposed to different lighting conditions: relationship with locomotor and feeding rhythms

In passerine birds, the periodic secretion of melatonin by the pineal organ represents an important component of the pacemaker that controls overt circadian functions. The daily phase of low melatonin secretion generally coincides with the phase of intense activity, but the precise relationship between the melatonin and the behavioral rhythms has not been studied. Therefore, we investigated in European starlings (Sturnus vulgaris) (1) the temporal relationship between the circadian plasma melatonin rhythm and the rhythms in locomotor activity and feeding; (2) the persistence of the melatonin rhythm in constant conditions; and (3) the effects of light intensity on synchronized and free-running melatonin and behavioral rhythms. There was a marked rhythm in plasma melatonin with high levels at night and/or the inactive phase of the behavioral cycles in almost all birds. Like the behavioral rhythms, the melatonin rhythm persisted for at least 50 days in constant dim light. In the synchronized state, higher daytime light intensity resulted in more tightly synchronized rhythms and a delayed melatonin peak. While all three rhythms usually assumed a rather constant phase relationship to each other, in one bird the two behavioral rhythms dissociated from each other. In this case, the melatonin rhythm retained the appropriate phase relationship with the feeding rhythm. Accepted: 10 December 1999     

Measurements of Light at Night (LAN) for a Sample of Female School Teachers

Epidemiological studies have shown an association between rotating shiftwork and breast cancer (BC) risk. Recently, light at night (LAN) measured by satellite photometry and by self-reports of bedroom brightness has been shown to be associated with BC risk, irrespective of shiftwork history. Importance has been placed on these associations because retinal light exposures at night can suppress the hormone melatonin and/or disrupt circadian entrainment to the local 24-h light-dark cycle. The present study examined whether it was valid to use satellite photometry and self-reports of brightness to characterize light, as it might stimulate the circadian system and thereby affect BC incidence. Calibrated photometric measurements were made at the bedroom windows and in the bedrooms of a sample of female school teachers, who worked regular dayshifts and lived in a variety of satellite-measured sky brightness categories. The light levels at both locations were usually very low and were independent of the amount of satellite-measured light. Calibrated photometric measurements were also obtained at the corneas of these female school teachers together with calibrated accelerometer measurements for seven consecutive days and evenings. Based upon these personal light exposure and activity measurements, the female teachers who participated in this study did not have disrupted light-dark cycles like those associated with rotating shiftworkers who do exhibit a higher risk for BC. Rather, this sample of female school teachers had 24-h light-dark and activity-rest patterns very much like those experienced by dayshift nurses examined in an earlier study who are not at an elevated risk of BC. No relationship was found between the amount of satellite-measured light levels and the 24-h light-dark patterns these women experienced. It was concluded from the present study that satellite photometry is unrelated to personal light exposures as they might affect melatonin suppression and/or circadian disruption. More generally, photometric devices calibrated in terms of the operational characteristics of the human circadian system must be used to meaningfully link LAN and BC incidence. (Authors correspondence: E-mail: ream@rpi.edu)     

Circadian mating activity and effect of pheromone pre-exposure on pheromone response rhythms in the moth Spodoptera littoralis

Mating in moths is generally mediated by female-produced sex pheromones. Mating activity, female pheromone production/release and male pheromone responsiveness all show diurnal variations in many species. We found that the response of the male Egyptian cotton leafworm, Spodoptera littoralis, to sex pheromone gland extracts showed a diel rhythm in olfactometer tests, and the variation was persistent for at least 1 day in constant darkness. High male response to sex pheromone was correlated in time with high mating and locomotor activity. Male S. littoralis, maintained in constant darkness and exposed to pheromone gland extracts on a daily basis, showed an induced temporal variation in response after several days, in contrast to unexposed males. This suggests that in the absence of other external zeitgebers, exposure to sex pheromone may function to synchronise circadian behavioural rhythms in male moths. The daily rhythm in mating activity in S. littoralis is also shown to be persistent for at least 2 days in constant darkness. Pairs mated significantly less when either the male or female had been raised in a light:dark cycle 10 h out of phase, indicating that the proposed circadian rhythm in mating activity is composed of rhythmic mating preference/ability in both sexes.     

Dim Light at Night Does Not Disrupt Timing or Quality of Sleep in Mice

Artificial nighttime illumination has recently become commonplace throughout the world; however, in common with other animals, humans have not evolved in the ecological context of chronic light at night. With prevailing evidence linking the circadian, endocrine, immune, and metabolic systems, understanding these relationships is important to understanding the etiology and progression of several diseases. To eliminate the covariate of sleep disruption in light at night studies, researchers often use nocturnal animals. However, the assumption that light at night does not affect sleep in nocturnal animals remains unspecified. To test the effects of light at night on sleep, we maintained Swiss-Webster mice in standard light/dark (LD) or dim light at night (DLAN) conditions for 8–10 wks and then measured electroencephalogram (EEG) and electromyogram (EMG) biopotentials via wireless telemetry over the course of two consecutive days to determine differences in sleep timing and homeostasis. Results show no statistical differences in total percent time, number of episodes, maximum or average episode durations in wake, slow-wave sleep (SWS), or rapid eye movement (REM) sleep. No differences were evident in SWS delta power, an index of sleep drive, between groups. Mice kept in DLAN conditions showed a relative increase in REM sleep during the first few hours after the dark/light transition. Both groups displayed normal 24-h circadian rhythms as measured by voluntary running wheel activity. Groups did not differ in body mass, but a marked negative correlation of body mass with percent time spent awake and a positive correlation of body mass with time spent in SWS was evident. Elevated body mass was also associated with shorter maximum wake episode durations, indicating heavier animals had more trouble remaining in the wake vigilance state for extended periods of time. Body mass did not correlate with activity levels, nor did activity levels correlate with time spent in different sleep states. These data indicate that heavier animals tend to sleep more, potentially contributing to further weight gain. We conclude that chronic DLAN exposure does not significantly affect sleep timing or homeostasis in mice, supporting the use of dim light with nocturnal rodents in chronobiology research to eliminate the possible covariate of sleep disruption.     

Circadian rhythms of agitation in institutionalized patients with Alzheimer's disease

Agitation is a common problem in institutionalized patients with Alzheimer's disease (AD). “Sundowning,” or agitation that occurs primarily in the evening, is estimated to occur in 10—25% of nursing home patients. The current study examined circadian patterns of agitation in 85 patients with AD living in nursing homes in the San Diego, California, area. Agitation was assessed using behavioral ratings collected every 15 minutes over 3 days, and activity and light exposure data were collected continuously using Actillume recorders. A five-parameter extension of the traditional cosine function was used to describe the circadian rhythms. The mean acrophase for agitation was 14:38, although there was considerable variability in the agitation rhythms displayed by the patients. Agitation rhythms were more robust than activity rhythms. Surprisingly, only 2 patients (2.4%) were“sundowners.”In general, patients were exposed to very low levels of illumination, with higher illumination during the night being associated with less robust agitation rhythms with higher rhythm minima (i.e., some agitation present throughout the day and night). Seasonality was examined; however, there were no consistent seasonal patterns found. This is the largest study to date to examine agitation rhythms using behavioral observations over multiple 24h periods. The results suggest that, although sundowning is uncommon, agitation appears to have a strong circadian component in most patients that is related to light exposure, sleep, and medication use. Further research into the understanding of agitation rhythms is needed to examine the potential effects of interventions targeting sleep and circadian rhythms. (Chronobiology International, 17(3), 405-418, 2000)     

Circadian rhythms in the blowfly, Phormia terraenovae: the period in constant light

ABSTRACT. Locomotor activity of individual blowflies, Phormia (=Protophormia) terraenovae R.D. (Diptera, Calliphoridae), was recorded by means of running wheels made of Perspex (plexiglass). At various intervals after hatching, flies were placed in the wheel in continuous light (120 lx). The period of the free-running rhythm was often shorter than 24 h initially, but lengthened later and stabilized after 15–20 days at values around or above 25 h. The mean period was independent of age in both sexes. Within a range from 2 to 2000lx the period did not depend in a systematic way on intensity of illumination; no activity could be recorded below 1 lx.     

Zeitgeber Hierarchy in Humans: Resetting the Circadian Phase Positions of Blind People Using Melatonin

Four blind individuals who were thought to be entrained at an abnormal circadian phase position were reset to a more normal phase using exogenous melatonin administration. In one instance, circadian phase was shifted later. A fifth subject who was thought to be entrained was monitored over four years and eventually was shown to have a circadian period different from 24 h. These findings have implications for treating circadian phase abnormalities in the blind, for distinguishing between abnormally entrained and free‐running blind individuals, and for informing the debate over zeitgeber hierarchy in humans.     

Circadian and Seasonal Responses in Indian Weaver Bird: Subjective Interpretation of Day and Night Depends Upon Both Light Intensity and Contrast Between Illuminations

This study investigated whether changes in illumination modify perception of day and night conditions in a diurnal species, the Indian weaver bird. Birds were initially subjected to a 12-h light:12-h dark regime (12L:12D; L?=?20 lux, D =?0.5 lux). After every 2 wks, the combinations of light illumination in L and D phases were changed as follows: 20:2 lux, 20:5 lux, 20:10 lux, 20:20 lux, 20:100 lux, and 20:200 lux. Finally, birds were released into dim constant light (0.5 lux) for 2 wks to determine the phase and period of the circadian activity rhythm. They were also laparotomized at periodic intervals to examine the effects of the light regimes on the seasonal testicular cycle. All individuals showed a consistently similar response. As evident by the activity pattern under these light regimes, both in total activity during contrasting light phases and during the 2?h in the beginning and end of first light phase, birds interpreted the period of higher light intensity as day, and the period of lower intensity as the night. During the period of similar light intensity, i.e., under LL, birds free-ran with a circadian period (～24?h). In bright LL (20 lux), the activity rhythm was less distinct, but periodogram analysis revealed the circadian period for the group as 24.46?±?0.41?h (mean?±?SE). However, in dim LL at the end of the experiment, all birds exhibited a circadian pattern with average period of 25.52?±?0.70?h. All birds also showed testicular growth and regression during the 16-wks study. It is suggested that weaver birds interpret day and night subjectively based on both the light intensity and contrast between illuminations during two phases over the 24?h. (Author correspondence: drskumar7@yahoo.com)     

Effects of meal timing on tumor progression in mice

Meal timing can reset circadian clocks in peripheral tissues. We investigated the effects of such non-photic entrainment on tumor growth rate. Two experiments involved a total of 61 male B6D2F(1) mice synchronized with an alternation of 12 h of light (L) and 12 h of darkness (D) (LD12:12). Mice were randomly allocated to have access to food ad libitum, or restricted to 4 or 6 h during L or D. Rest-activity and body temperature, two circadian outputs, were monitored with an intra-peritoneal sensor. Glasgow osteosarcoma was inoculated into both flanks of each mouse ten days after meal timing onset. Before tumor inoculation, meal timing during D amplified the 24-h rhythms in rest-activity and body temperature with minimal phase alteration as compared to ad libitum feeding. Conversely, meal timing during L induced dominant 12-h or 8-h rhythmic components in activity, nearly doubled the 24-h amplitude of body temperature and shifted its acrophase (time of maximum) from approximately mid-D to approximately mid-L. Thirteen days after tumor inoculation, mean tumor weight (+/- SEM, mg) was 1503 +/- 150 in ad libitum mice, 1077 +/- 157 in mice fed during D and 577 +/- 139 in mice fed during L (ANOVA, p < 0.0001). Overall survival was prolonged in the mice fed during L (median, 17.5 days, d) as compared with those fed during D (14.5 d) or ad libitum (14 d) (Log Rank, p = 0.0035). The internal desynchronization produced by meal timing during L slowed down tumor progression, an effect possibly resulting from improved host-mediated tumor control and/or altered tumor circadian clocks.     

Phase relations between the circadian leaf movement rhythm and its zeitgeber within the range of entrainment in the cotton plant,Gossypium hirsutum L.

Abstract

The leaf movement rhythm of Gossypium hirsutum L. (cv. Lakshmi) could be entrained to 24 h LD cycles with different photofractions varying from 4 to 20 h such that the night peak position of the rhythm occurred during darkness. The phase angle (ψ) of the rhythm varied in a regular manner with different photoperiods of a 24 h LD cycle. Under 24 h LD cycles with different photoperiods, the leaf movement shows probable evidences for the concurrent participation of a ‘light‐on’ and ‘light‐off rhythm.     

Effects of twilights on circadian entrainment patterns and reentrainment rates in squirrel monkeys

Entrainment patterns of the circadian rhythms of body temperature and locomotor activity were compared in 6 squirrel monkeys (Saimiri sciureus) exposed to daily illumination cycles with abrupt transitions between light and darkness (LD-rectangular) or with gradual dawn and dusk transitions simulating natural twilights at the equator (LD-twilight). Daytime light intensity was 500 lux, and the total amount of light emitted per day was the same in the two conditions. Mean daytime body temperature levels were stable in LD-rectangular but increased gradually in LD-twilight, reaching peak levels during the dusk twilight. Locomotor activity showed a similar pattern, but with an additional, secondary peak near the end of dawn. Activity duration was about 0.5 h longer in LD-twilight than in LD-rectangular, but the time of activity midpoint was similar in the two conditions. Reentrainment of the body temperature rhythm was faster following an 8-h advance of the LD cycle than following an 8-h delay, but did not differ significantly between the two LD conditions. These results provide no evidence that the inclusion of twilight transitions affected the strength of the LD Zeitgeber, and suggest that the observed differences in the daily patterns reflected direct effects of light intensity on locomotor activity and body temperature rather than an effect of twilights on circadian entrainment mechanisms.Abbreviation LD light-dark     

Effect of Light Intensity on the Phase and Period Response Curves in the Nocturnal Field Mouse Mus booduga

The effect of light intensity on the phase response curve (PRC) and the period response curve (τRC) of the nocturnal field mouse Mus booduga was studied. PRCs and τRCs were constructed by exposing animals free-running in constant darkness (DD), to fluorescent light pulses (LPs) of 100 lux and 1000 lux intensities for 15min duration. The waveform of the PRCs and τRCs evoked by high light intensity (1000 lux) stimuli was significantly different compared to those constructed using low light intensity (100 lux). Moreover, a weak but significant correlation was observed between phase shifts and period changes when light stimuli of 1000 lux intensity were used; however, the phase shifts and period changes in the 100 lux PRC and τRC were not correlated. This suggests that the intensity of light stimuli affects both phase and period responses in the locomotor activity rhythm of the nocturnal field mouse M. booduga. These results indicate that complex mechanisms are involved in entrainment of circadian clocks, even in nocturnal rodents, in which PRC, τRC, and dose responses play a significant role.     

Effects of One Night of Partial Sleep Deprivation upon Diurnal Rhythms of Accuracy and Consistency in Throwing Darts

Sixty subjects were tested five times per waking day on two occasions for accuracy and reliability in throwing 20 darts at a target. Two experimental conditions were investigated: following a normal nocturnal sleep (7–8 h sleep, normal) and after having retired to bed 4 h later than normal the previous night but rising at the normal time (3–4 h sleep, sleep deprivation). Sublingual (core) temperature and subjective estimates of alertness and fatigue were measured in all sessions. Performance at throwing darts was assessed by three methods: mean distance of the dart from the bulls-eye; number of times the target was missed; and variability of the scores from the darts thrown. There was no evidence that performance was affected by physical fatigue arising during the course of throwing the 20 darts. All variables showed significant diurnal rhythms, those of alertness and performance being phased over 1 h earlier than core temperature, and that of fatigue over 1 h earlier than the inverse of temperature. Core temperature was not affected by sleep deprivation, but all other variables showed significant changes, indicative of mood and performance decrement. Increasing time awake was associated with decreased alertness and increased fatigue, as well as slight negative effects upon performance. We conclude that the simple task of throwing darts at a target provides information about chronobiological changes in circumstances where time awake and sleep loss might affect psychomotor performance. (Author correspondence: b.j.edwards@ljmu.ac.uk)