Socially Adjusted Synchrony in the Activity Profiles of Common Marmosets in Light-Dark Conditions

Synchronized state of activity and rest might be attained by mechanisms of entrainment and masking. Most zeitgebers not only act to entrain but also to mask circadian rhythms. Although the light-dark (LD) cycle is the main zeitgeber of circadian rhythms in marmosets, social cues can act as weaker zeitgebers. Evidence on the effects of social entrainment in marmosets has been collected in isolated animals or in pairs where activity is not individually recorded. To characterize the synchronization between the daily activity profiles of individuals in groups under LD conditions, the motor activity of animals from five groups was continuously monitored using actiwatches for 15 days during the 5th, 8th, and 11th months of life of juveniles. Families consisting of twins (4 ♂♀/1 ♂♂) and their parents were maintained under controlled lighting (LD 12:12?h), temperature, and humidity conditions. Synchronization was evaluated through the synchrony between the circadian activity profiles obtained from the Pearson correlation index between possible pairs of activity profiles in the light and dark phases. We also calculated the phase-angle differences between the activity onset of one animal in relation to the activity onset of each animal in the group (ψ_on). A similar procedure was performed for activity offset (ψ_off). By visual analysis, the correlation between the activity profiles of individuals within each family was stronger than that of individuals from different families. A mixed-model analysis showed that within the group, the correlation was stronger between twins than between twins and their parents in all families, except for the family in which both juveniles were males. Because a twin is an important social partner for juveniles, a sibling is likely to have a stronger influence on its twin’s activity rhythm than other family members. Considering only the light phase, the second strongest correlation was observed between the activity profiles of the individuals in the reproductive pair. Regarding the parameters ψ_on and ψ_off, the juvenile/juvenile dyad had lower values than the other dyads, but these differences did not reach statistical significance in relation to all dyads. Comparing the results of the ψ_on and ψ_off, and correlation indices, we suggest that the latter could detect differences between the animals that were not observed in the results of the phase-angle differences. These differences could be related to changes that occur during the active phase but not only in a particular phase, such as the temporal changes during the activity phase that characterize unimodal or bimodal patterns. Based on the differences in the correlations between individuals subjected to the same LD routine, we suggest that social cues modulate the circadian activity profiles of marmosets as a result of interactions between the animals within each group. Future studies are necessary to characterize the mechanisms of synchronization that are involved in this social modulation. (Author correspondence: carolina@cb.ufrn.br)     

Aging-related changes on social synchronization of circadian activity rhythm in a diurnal primate (Callithrix jacchus)

ABSTRACT

The input of environmental time cues and expression of circadian activity rhythms may change with aging. Among nonphotic zeitgebers, social cues from conspecific vocalizations may contribute to the stability and survival of individuals of social species, such as nonhuman primates. We evaluated aging-related changes on social synchronization of the circadian activity rhythm (CAR) in a social diurnal primate, the common marmoset. The activity of 18 male marmosets was recorded by actiwatches in two conditions. (1) Experimental – 4 young adult (5 ± 2 yrs of age) and 4 older (10 ± 2 yrs of age) animals maintained under LD 12/12 h and LL in a room with full insulation for light but only partial insulation for sound from vocalizations of conspecifics maintained outdoors in the colony; and (2) Control – 10 young adult animals maintained outdoors in the colony (5 animals as a control per age group). In LL, the CAR of young adults showed more stable synchronization with controls. Among the aged marmosets, two free-ran with τ > 24 h, whereas the other two showed relative coordination during the first 30 days in LL, but free-ran thereafter. These differences were reflected in the “social” phase angles (ψ_on and ψ_off ) between rhythms of experimental and control animal groups. Moreover, the activity patterns of aged animals showed lower social synchrony with controls compared to young adults, with the time lags of the time series between each experimental group and control group being negative in aged and positive in young adult animals (t-test, p < 0.05). The index of stability of the CAR showed no differences according to age, while the intradaily variability of the CAR was higher in the aged animals during LD-resynchronization, who took additional days to resynchronize. Thus, the social modulation on CAR may vary with age in marmosets. In the aged group, there was a lower effect of social synchronization, which may be associated with aging-related changes in the synchronization and generation of the CAR as well as in system outputs.     

Forced desynchronization model for a diurnal primate

The circadian system is organized in a hierarchy of multiple oscillators, with the suprachiasmatic nucleus (SCN) as the master oscillator in mammals. The SCN is formed by a group of coupled cell oscillators. Knowledge of this coupling mechanism is essential to understanding entrainment and the expression of circadian rhythms. Some authors suggest that light-dark (LD) cycles with periods near the limit of entrainment may be good models for promoting internal desynchronization, providing knowledge about the coupling mechanism. As such, we evaluated the circadian activity rhythm (CAR) pattern of marmosets in LD cycles at lower limits of entrainment in order to study induced internal dissociation. To that end, two experiments were conducted: (1) 6 adult females were under symmetrical LD cycles T21, T22 and T21.5 for 60, 35 and 48 days, respectively; and (2) 4 male and 4 female adults were under T21 for 24 days followed by 18 days of LL, back to T21 for 24 days, followed by 14 days of LL. The CAR of each animal was continuously recorded. In experiment 1, vocalizations were also recorded. Under Ts shorter than 24 days, a dissociation pattern was observed for CAR and vocalizations. Two simultaneous circadian components emerged, one with the same period as the LD cycle, called the light-entrained component, and the other in free-running, denominated the non-light-entrained component. Both components were displayed in the CAR for all the animals in T21, five animals (83.3%) in T21.5 and two animals (33.3%) in T22. Our results are in accordance with the multioscillatory nature of the circadian system. Dissociation is partial synchronization to the LD cycle, with at least one group of oscillators synchronized by relative coordination and masking, while another group of oscillators free runs, but is also masked by the LD cycle. Since only T21 promoted the emergence of both circadian components in the circadian rhythms of all marmosets, it was considered the promoter period of circadian rhythm dissociation in this species, and is proposed as a good animal model for forced desynchronization in non-human diurnal primates.     

Photic and nonphotic circadian phase resetting in a diurnal primate, the common marmoset

Despite the considerable literature on circadian entrainment, there is little information on this subject in diurnal mammals. Contributing to this lack of understanding is the problem of separating photic from nonphotic (behavioral) phase-resetting events in diurnal species. In the present study, photic phase resetting was obtained in diurnal common marmosets held under constant dim light (DimDim; <0.5 lx) by using a 20-s pulse of bright light to minimize time available for behavioral arousal. This stimulus elicited phase advances at circadian time (CT) 18-22 and phase delays at CT9-12. Daily presentation of these 20-s pulses produced entrainment with a phase angle of approximately 11 h (0 h = activity onset). Nonphotic phase resetting was obtained under DimDim with the use of a 1-h-induced activity pulse, consisting of intermittent cage agitation and water sprinkling, delivered in total darkness to minimize photic effects. This stimulus caused phase delays at CT20-24, and entrainment to a scheduled daily regimen of these pulses occurred with a phase angle of approximately 0 h. These results indicate that photic and nonphotic phase-response curves (PRCs) of marmosets are similar to those of nocturnal rodents and that nonphotic PRCs are keyed to the phase of the suprachiasmatic nucleus pacemaker, not to the phase of the activity-rest cycle.     

The effects on heart rate and temperature of mice and vas deferens responses to noradrenaline when their cage mates are subjected to daily restraint stress

Performing stressful procedures in view of cage mates may cause stress in observer animals. However, it is not known if stressful procedures performed in close proximity to, but not in view of cage mates are stressful for the (observer) cage mates. Radiotelemetry and postmortem in vitro studies of the vas deferens were used to determine the effects of stress on observers. Heart rate (HR) and core body temperature (cBT) were recorded for 1 h following weighing of a cage mate or 1 h during restraint of a cage mate and the hour following return of the restrained mouse to the cage. This procedure was repeated daily for 15 days. HR and cBT were increased in observers during both restraint and weighing of cage mates. Analysis of the area under the curve showed that HR and cBT in observers were significantly higher during restraint of a cage mate than after weighing of a cage mate. When mice were returned to the cage after weighing or restraint, HR and cBT were significantly higher in the cage mates of restrained animals. Comparison between days 1, 3, 7 and 14 found that, as the experiment progressed, HR and cBT were significantly reduced in the observer mice during the hour following return of the cage mates after restraint. Results from previous studies have shown that chronic stress causes the vas deferens to become hypersensitive to exogenous application of noradrenaline (NAd). In this study, vas deferens from observers of restraint had a significantly increased response to NAd. These results indicate that stressful procedures should be conducted in isolation from other mice.     

Changes in phase-angle under light–dark cycles influenced by nonphotic stimulation

ABSTRACT

Most work looking at nonphotic effects on circadian rhythms is conducted when animals are held under freerunning conditions, usually constant darkness. However, for nonphotic effects to be functionally significant, they should be demonstrable under conditions in which most animals live, i.e., a 24-hr light–dark cycle (LD). Syrian hamsters held in LD 6:18 were administered nonphotic stimulation in the form of a 3-hr confinement to a novel wheel starting about 6 hr before the start of their normal nightly activity bout. This resulted in a 2.5-hr advance of their activity rhythm on the next day that gradually receded to about 1.5 hr over the next 10 days. When hamsters held in LD 6:18 were given five novel wheel confinements over 13 days their nightly activity onset advanced 3 hr and remained at that phase for at least 2 weeks. Home cage wheel deprivation experiments indicated that high levels of home cage activity are necessary to maintain the advanced phase. These results show that nonphotic stimulation can have large, long-lasting effects on daily rhythms in LD and suggest a possible mechanism whereby nocturnal rodents might achieve phase flexibility in response to seasonal changes.     

Locomotor Activity Rhythm in Free-ranging Common Marmosets (Callithrix jacchus)

Locomotor activity rhythm in the common marmoset has been well described and characterized in captivity, but there is no information about rhythmic aspects of this behavior in free-ranging conditions. To investigate if the locomotor activity rhythm in free-ranging common marmosets is similar to that found in captive common marmosets, we observed reproductive animals from two groups that inhabited at the Experimental Forestry Station, in northeastern Brazil (6°5'S, 35°12'W). From December 1992 to October 1993 we recorded the frequency of locomotor activity by focal animal sampling. The records of locomotor activity were submitted to spectral analysis and to the Cosinor test. Common marmosets showed an average duration of the active phase of 11:37 h ± 13.8 min. Similar to recorded in captive common marmosets, circadian rhythmicity was found with peaks in the morning, and in the afternoon, characterizing a bimodal pattern with peaks separated by 8 hours. The presence of a rhythmic pattern in the locomotor activity in free-ranging and captive common marmosets suggests that this pattern is not determined by the environmental conditions in captivity, but it has a endogenous component that play an important role for adaptation of this species to different conditions.     

Time course of vocal modulation during isolation in common marmosets (Callithrix jacchus)

Common marmosets vocalize phee calls as isolation calls, which seem to facilitate their reunion with family groups. To identify multiple acoustic properties with different time courses, we examined acoustic modulations of phees during different social contexts of isolation. Subject marmosets were totally isolated in one condition, were visually isolated and could exchange vocalizations in another condition, and were visually isolated and subsequently totally isolated in a third condition. We recorded 6,035 phees of 10 male–female marmoset pairs and conducted acoustic analysis. The marmosets frequently vocalized phees that were temporally elongated and louder during isolation, with varying time courses of these changes in acoustic parameters. The vocal rates and sound levels of the phees increased as soon as the marmosets saw their pair mates being taken away, and then gradually calmed down. The phee duration was longer in conditions during which there were no vocal responses from their pair mates. Louder vocalizations are conspicuous and seem to be effective for long‐distance transmission, whereas shorter call duration during vocal exchanges might avoid possible vocal overlap between mates. Am. J. Primatol. 72:681–688, 2010. © 2010 Wiley‐Liss, Inc.     

An experimental investigation of the pair bond in the callitrichid monkey,Saguinus labiatus

The strength and specificity of attraction between mated pairs of red-bellied tamarins (Saguinus labiatus) were tested in the laboratory. Test individuals were placed in a choice situation in which they could indicate their preference, by proximity and quality of behavior measures, to either their pair mate in an adjoining cage or an alternative in a second adjoining cage (same-sex animal, opposite-sex animal, or an empty cage). The test animals showed a significant preference to be in visual contact and proximity with their pair mates instead of the alternative when it was an animal of the same sex as the test individual or an empty cage. However, when the choice was between their pair mate and an animal of the opposite sex, no preference for their pair mate was apparent. No sex differences were found. It is concluded that attraction to a pair mate is not sufficient to maintain a pair bond, and when pairs ofS. labiatus breed monogamously, the pair relationship is likely to be maintained, at least in part, through aggression between same-sex individuals.     

Melatonin-induced phase and dose responses in a diurnal mammal,Funambulus pennantii

ABSTRACT

Melatonin, an essential pineal hormone, acts as a marker of the circadian clock that regulates biological rhythms in animals. The effects of exogenous melatonin on the circadian system of nocturnal rodents have been extensively studied; however, there is a paucity of studies on the phase-resetting characteristics of melatonin in diurnal rodents. We studied the phase shifting effects of exogenous melatonin as a single melatonin injection (1 mg/kg) at various phases of the circadian cycle on the circadian locomotor activity rhythm in the palm squirrel, Funambulus pennantii. A phase response curve (PRC) was constructed. Adult male squirrels (N = 10) were entrained to a 12:12 h light-dark cycle (LD) in a climate-controlled chronocubicle with food and water provided ad libitum. After stable entrainment, squirrels were transferred to constant dark condition (DD) for free-running. Following stable free run, animals were administered a single dose of melatonin (1 mg/kg in 2% ethanol-phosphate buffered saline (PBS) solution) or vehicle (2% ethanol-PBS solution) at circadian times (CTs) 3 h apart to evoke phase shifts. The phase shifts elicited at various CTs were plotted to generate the PRC. A dose response curve was generated using four doses (0.5, 1, 2 and 4 mg/kg) administered at the CT of maximum phase advance. Melatonin evoked maximum phase advances at CT0 (1.23 ± 0.28 h) and maximum phase delays at CT15 (0.31 ± 0.09 h). In the dose response experiment, maximal phase shifts were evoked with 1 mg/kg. In contrast, no significant shifts were observed in control groups. Our study demonstrates that the precise timing and appropriate dose of melatonin administration is essential to maximize the amelioration of circadian rhythm–related disorders in a diurnal model.     

Evidence for age-related changes in the circadian activity rhythm of the diurnal primate Callithrix jacchus: a case report

Many cross-sectional studies have shown that circadian rhythms change with age, but such age-related modifications are gradual and may be insufficiently described by cross-sectional studies. In this case study, circadian activity rhythm (CAR) was evaluated longitudinally, in both LD (12:12) and LL conditions, on two occasions in a single male marmoset: when ‘adult’ (3 y.o.) and when ‘old’ (9 y.o.). When adult, the CAR synchronized with positive phase angles for the onset and offset of activity. In LL, the rhythm free-ran with τ < 24 h. When old, the animal showed a significant phase delay of its activity rhythm with respect to the LD cycle (t-test, p < 0.01) and a reduction on total daily activity (t-test, p < 0.01), with signs of lesser stability, greater fragmentation and some loss of photic synchronization. In LL, the period free-ran with τ > 24 h. We conclude that aging is associated with attenuation of photic synchronization and expression of CAR in LL in the marmoset. Further studies with a larger number of individuals are needed to confirm these findings.     

Euclidean distances discriminatively predict short-term and long-term attraction to potential mates

We tested the ability of a Euclidean algorithm to predict attraction to potential mates—a relatively upstream domain in the temporal sequence of the mating process. Participants in two studies reported their ideal mate preferences using a 23-item preference instrument. Separately, they rated their attraction to profiles of potential mates that varied on those 23 dimensions. Study 1 (N = 522) found that Euclidean distances predicted attraction to potential mates both in terms of (1) overall mate value and (2) unique mate value. Study 2 (N = 411) replicated these effects and further found that Euclidean mate values discriminatively predict between short- and long-term attraction. Across both studies, a Euclidean model outperformed a variety of alternative models for predicting attraction to potential mates. These results suggest that a Euclidean algorithm is a good model for how multiple preferences are integrated in mate choice.     

Social Interactions and the Circadian Rhythm in Locomotor Activity in the Cockroach Leucophaea maderae

The role of social interactions in entrainment has not been extensively studied in the invertebrates. Leucophaea maderae is a gregarious species of cockroach that exhibits extensive social interactions. Social interactions associated with copulation between the sexes have been shown to be regulated by the circadian system. We show here that social interactions between males are also under circadian control. We examined the question of whether or not these rhythmic social contacts could function as zeitgebers capable of regulating circadian phase and period. Animals initially in phase that were housed as groups or pairs of single sex or mixed sex in constant darkness for 2–7 weeks were found to drift out of phase. Their behavior was not significantly different from individual animals maintained in isolation. Further, animals that were initially out of phase by 12 h housed as groups or pairs were not significantly different in phase from animals that were isolated. The results show the circadian clocks of cockroaches are remarkably insensitive to the extensive social interactions that occur between individuals.     

Pygmy Marmosets Modify Call Structure When Paired

Pygmy marmosets ( Cebuella pygmaea ) modified the structure of their trill vocalizations in response to pairing with a new mate. Prior to pairing, individual marmosets displayed different acoustic parameters of trill structure. Within the first 6 wk after pairing, three of four pairs produced trills that were more homogeneous between mates. Pairs that had the greatest individual difference in trill structure prior to pairing displayed convergence on a greater number of parameters than pairs having fewer differences in prepairing trill structure. The vocal structure following pairing remained highly stable in the two pairs that could be recorded 3 yr after initial pairing. Changes in trill structure that occurred within the first 6 wk after pairing were greater than spontaneous changes that occurred during baseline control recording sessions on the same animals. These results suggest that nonhuman primates show subtle modification of vocal structure, especially in response to changes in social environment. Similar findings have been reported in birds and other mammals.     

Effect of ambient temperature on the circadian activity rhythm in common marmosets, Callithrix j. jacchus (primates)

Whereas the (zeitgeber) effect of ambient temperature T_a and temperature cycles TaC's on circadian rhythmicity has been well documented for heterofhermic mammals, inconsistent results have been obtained for strictly homeothermic species. Hence, it might be inferred that the susceptibility of the mammalian circadian timing system (CTS) to Ta and TaC's depends on the range of the animals' core and/or brain temperature rhythm. This hypothesis was tested in the common marmoset (Callithrix j. jacchus, n=12), a small diurnal primate with an amplitude in body temperature rhythm that is larger than for other homeothermic primates studied so far. Within the range 20-30°C, no systematic effects of constant Ta on most parameters of the marmosets' light-dark (LD)-entrained and free-running circadian activity rhythm (CAR) were found. Significant differences could be established in the average amount of activity per circadian cycle. It was highest at Ta 25°C (LD) and 20°C (light-light, LL) and most probably reflected a temperature-induced masking effect. A 24h trapezoidal TaC of 20:30°C entrained the free-running CAR in two of six marmosets and produced relative coordination in all others. Accordingly, in all animals tested, it had an effect on the CTS. In marmosets free running in LL at a Ta of 20°C or 30°C, 3h warm and cold pulses of 30°C and 20°C, respectively, produced neither systematic phase responses nor period responses of the CAR. So, there is no evidence of a phase-response mechanism underlying circadian entrainment. The results show that large-amplitude TaC's function as a weak zeitgeber for the marmosets' CTS. Since this zeitgeber effect is significantly larger than that found in owl monkeys, the results are consistent with the starting hypothesis that the zeitgeber effect of a given T,C on the mammalian CTS may be related to the amplitude of the species' core and/or brain temperature cycle.     

Kinship alters the effects of forced cohabitation on body weight,mate choice and fitness in the rat-like hamster Tscheskia triton

It has been documented that social isolation imparts deleterious effects on gregarious rodents species,but caging in group imparts such effects on solitary rodents. This study was attempted at examining how kinship to affect body weight,behavioral interaction,mate choice and fitness when we caged male and female rat-like hamsters Tscheskia triton in pair,a solitary species. We found that females paired with nonsibling males became heavier than the females paired with sibling males,but both agonistic and amicable behavior between paired males and females did not differ between sibling and nonsibling groups. This indicated that kinship might reduce females' obesity in response to forced cohabitation,and dissociation might exist between physiological and behavioral responses. Furthermore,binary choice tests revealed that social familiarity between either siblings or nonsiblings decreased their investigating time spent in opposite sex conspecific of cage mates and/or their scents as compared with those of nonmates,suggesting effects of social association on mate and kin selection of the hamsters. On the other side,both females and males caged in pair with siblings show a preference between unfamiliar siblings or their scents and the counterparts of nonsiblings after two month separation,indicating that the kin recognition of the hamsters might also rely on phenotype matching. In addition,cohabitation (or permanent presence of fathers) elicited a lower survival of pups in nonsibling pairs than sibling pairs,but did not affect litter size,suggesting that kinship affects fitness when housing male and female ratlike hamsters together. Therefore,inbreeding might be adapted for rare and endangered animals.     

A novel method for activity monitoring in small non-human primates

Patterns of spontaneous activity are valuable reflections of well-being in animals and humans and, because of this, investigations have frequently incorporated some form of activity monitoring into their studies. It is widely believed that activity monitoring, alongside assessments of general behaviour, should be included in initial CNS safety pharmacology screening. As the number of marmoset studies having actimetry as their focus, or as an adjunct, is increasing, we wished to evaluate an alternative approach to those commonly used. The method is based on miniaturized accelerometer technologies, currently used for human activity monitoring.Actiwatch-Minis were used to monitor the activity of two groups of differently housed marmosets for 14 consecutive days. Group A consisted of four mixed-sex pairs of animals and group B comprised eight group-housed males. Activity profiles were generated for weekday and weekend periods. The devices captured quantifiable data which showed differences in total activity between the two differently housed groups and revealed intragroup variations in the temporal spread of activity between weekdays and weekends. The Actiwatch-Mini has been shown to generate retrospective, data-logged activity counts recorded from multiple animals in a single arena by means of non-invasive monitoring.     

Temperature-independence of circannual variations in circadian rhythms of golden-mantled ground squirrels

In golden-mantled ground squirrels, phase angles of entrainment of circadian locomotor activity to a fixed light-dark cycle differ markedly between subjective summer and winter. A change in ambient temperature affects entrainment only during subjective winter when it also produces pronounced effects on body temperature (Tb). It was previously proposed that variations in Tb are causally related to the circannual rhythm in circadian entrainment. To test this hypothesis, wheel-running activity and Tb were monitored for 12 to 14 months in castrated male ground squirrels housed in a 14:10 LD photocycle at 21 degrees C. Animals were treated with testosterone implants that eliminated hibernation and prevented the marked winter decline in Tb; these squirrels manifested circannual changes in circadian entrainment indistinguishable from those of untreated animals. Both groups exhibited pronounced changes in phase angle and alpha of circadian wheel-running and Tb rhythms. Seasonal variation in Tb is not necessary for circannual changes in circadian organization of golden-mantled ground squirrels.     

Photic phase response curve in Octodon degus: assessment as a function of activity phase preference

Light exposure during the early and late subjective night generally phase delays and advances circadian rhythms, respectively. However, this generality was recently questioned in a photic entrainment study in Octodon degus. Because degus can invert their activity phase preference from diurnal to nocturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activity phase preference in a 24-h light-dark cycle (LD 12:12), degus were released in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many circadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-24: n = 7). The circadian phase of body temperature (Tb) onset was defined as CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when delivered during the early (CT 13-16) and late (CT 20-23) subjective night, respectively. No significant phase shifts were observed during the middle of the subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar to most other diurnal and nocturnal species, suggesting that entrainment mechanisms do not determine overt diurnal and nocturnal behavior.     

Circadian phase,circadian period and chronotype are reproducible over months

The timing of the circadian clock, circadian period and chronotype varies among individuals. To date, not much is known about how these parameters vary over time in an individual. We performed an analysis of the following five common circadian clock and chronotype measures: 1) the dim light melatonin onset (DLMO, a measure of circadian phase), 2) phase angle of entrainment (the phase the circadian clock assumes within the 24-h day, measured here as the interval between DLMO and bedtime/dark onset), 3) free-running circadian period (tau) from an ultradian forced desynchrony protocol (tau influences circadian phase and phase angle of entrainment), 4) mid-sleep on work-free days (MSF from the Munich ChronoType Questionnaire; MCTQ) and 5) the score from the Morningness–Eveningness Questionnaire (MEQ). The first three are objective physiological measures, and the last two are measures of chronotype obtained from questionnaires. These data were collected from 18 individuals (10 men, eight women, ages 21–44 years) who participated in two studies with identical protocols for the first 10 days. We show how much these circadian rhythm and chronotype measures changed from the first to the second study. The time between the two studies ranged from 9 months to almost 3 years, depending on the individual. Since the full experiment required living in the laboratory for 14 days, participants were unemployed, had part-time jobs or were freelance workers with flexible hours. Thus, they did not have many constraints on their sleep schedules before the studies. The DLMO was measured on the first night in the lab, after free-sleeping at home and also after sleeping in the lab on fixed 8-h sleep schedules (loosely tailored to their sleep times before entering the laboratory) for four nights. Graphs with lines of unity (when the value from the first study is identical to the value from the second study) showed how much each variable changed from the first to the second study. The DLMO did not change more than 2 h from the first to the second study, except for two participants whose sleep schedules changed the most between studies, a change in sleep times of 3 h. Phase angle did not change by more than 2 h regardless of changes in the sleep schedule. Circadian period did not change more than 0.2 h, except for one participant. MSF did not change more than 1 h, except for two participants. MEQ did not change more than 10 points and the categories (e.g. M-type) did not change. Pearson’s correlations for the DLMO between the first and second studies increased after participants slept in the lab on their individually timed fixed 8-h sleep schedules for four nights. A longer time between the two studies did not increase the difference between any of the variables from the first to the second study. This analysis shows that the circadian clock and chronotype measures were fairly reproducible, even after many months between the two studies.