Clock-gated photic stimulation of timeless expression at cold temperatures and seasonal adaptation in Drosophila

Drosophila photoreceptor systems converge in arousal neurons and confer light responsive robustness

Lateral ventral neurons (LNvs) in the fly circadian neural circuit mediate behaviors other than clock resetting, including light-activated acute arousal. Converging sensory inputs often confer functional redundancy. The LNvs have three distinct light input pathways: (1) cell autonomously expressed cryptochrome (CRY), (2) rhodopsin 7 (Rh7), and (3) synaptic inputs from the eyes and other external photoreceptors that express opsins and CRY. We explored the relative photoelectrical and behavioral input contributions of these three photoreceptor systems to determine their functional impact in flies. Patch-clamp electrophysiology measuring light evoked firing frequency (FF) was performed on large LNvs (l-LNvs) in response to UV (365 nm), violet (405 nm), blue (450 nm), or red (635 nm) LED light stimulation, testing controls versus mutants that lack photoreceptor inputs gl60j, cry-null, rh7-null, and double mutant gl60j-cry-null flies. For UV, violet, and blue short wavelength light inputs, all photoreceptor mutants show significantly attenuated action potential FF responses measured in the l-LNv. In contrast, red light FF responses are only significantly attenuated in double mutant gl60j-cry-null flies. We used a light-pulse arousal assay to compare behavioral responses to UV, violet, blue and red light of control and light input mutants, measuring the awakening arousal response of flies during subjective nighttime at two different intensities to capture potential threshold differences (10 and 400 μW/cm2). The light arousal behavioral results are similar to the electrophysiological results, showing significant attenuation of behavioral light responses for mutants compared to control. These results show that the different LNv convergent photoreceptor systems are integrated and together confer functional redundancy for light evoked behavioral arousal.

Roles of peripheral clocks: lessons from the fly

To adapt to and anticipate rhythmic changes in the environment such as daily light-dark and temperature cycles, internal timekeeping mechanisms called biological clocks evolved in a diverse set of organisms, from unicellular bacteria to humans. These biological clocks play critical roles in organisms' fitness and survival by temporally aligning physiological and behavioral processes to the external cues. The central clock is located in a small subset of neurons in the brain and drives daily activity rhythms, whereas most peripheral tissues harbor their own clock systems, which generate metabolic and physiological rhythms. Since the discovery of Drosophila melanogaster clock mutants in the early 1970s, the fruit fly has become an extensively studied model organism to investigate the mechanism and functions of circadian clocks. In this review, we primarily focus on D. melanogaster to survey key discoveries and progresses made over the past two decades in our understanding of peripheral clocks. We discuss physiological roles and molecular mechanisms of peripheral clocks in several different peripheral tissues of the fly.

Perception of Daily Time: Insights from the Fruit Flies

We create mental maps of the space that surrounds us; our brains also compute time—in particular, the time of day. Visual, thermal, social, and other cues tune the clock-like timekeeper. Consequently, the internal clock synchronizes with the external day-night cycles. In fact, daylength itself varies, causing the change of seasons and forcing our brain clock to accommodate layers of plasticity. However, the core of the clock, i.e., its molecular underpinnings, are highly resistant to perturbations, while the way animals adapt to the daily and annual time shows tremendous biological diversity. How can this be achieved? In this review, we will focus on 75 pairs of clock neurons in the Drosophila brain to understand how a small neural network perceives and responds to the time of the day, and the time of the year.

The role of clockwork orange in the circadian clock of the cricket Gryllus bimaculatus

The circadian clock generates rhythms of approximately 24 h through periodic expression of the clock genes. In insects, the major clock genes period (per) and timeless (tim) are rhythmically expressed upon their transactivation by CLOCK/CYCLE, with peak levels in the early night. In Drosophila, clockwork orange (cwo) is known to inhibit the transcription of per and tim during the daytime to enhance the amplitude of the rhythm, but its function in other insects is largely unknown. In this study, we investigated the role of cwo in the clock mechanism of the cricket Gryllus bimaculatus. The results of quantitative RT-PCR showed that under a light/dark (LD) cycle, cwo is rhythmically expressed in the optic lobe (lamina-medulla complex) and peaks during the night. When cwo was knocked down via RNA interference (RNAi), some crickets lost their locomotor rhythm, while others maintained a rhythm but exhibited a longer free-running period under constant darkness (DD). In cwo^RNAi crickets, all clock genes except for cryptochrome 2 (cry2) showed arrhythmic expression under DD; under LD, some of the clock genes showed higher mRNA levels, and tim showed rhythmic expression with a delayed phase. Based on these results, we propose that cwo plays an important role in the cricket circadian clock.

CRTC Potentiates Light-independent timeless Transcription to Sustain Circadian Rhythms in Drosophila

Light is one of the strongest environmental time cues for entraining endogenous circadian rhythms. Emerging evidence indicates that CREB-regulated transcription co-activator 1 (CRTC1) is a key player in this pathway, stimulating light-induced Period1 (Per1) transcription in mammalian clocks. Here, we demonstrate a light-independent role of Drosophila CRTC in sustaining circadian behaviors. Genomic deletion of the crtc locus causes long but poor locomotor rhythms in constant darkness. Overexpression or RNA interference-mediated depletion of CRTC in circadian pacemaker neurons similarly impairs the free-running behavioral rhythms, implying that Drosophila clocks are sensitive to the dosage of CRTC. The crtc null mutation delays the overall phase of circadian gene expression yet it remarkably dampens light-independent oscillations of TIMELESS (TIM) proteins in the clock neurons. In fact, CRTC overexpression enhances CLOCK/CYCLE (CLK/CYC)-activated transcription from tim but not per promoter in clock-less S2 cells whereas CRTC depletion suppresses it. Consistently, TIM overexpression partially but significantly rescues the behavioral rhythms in crtc mutants. Taken together, our data suggest that CRTC is a novel co-activator for the CLK/CYC-activated tim transcription to coordinate molecular rhythms with circadian behaviors over a 24-hour time-scale. We thus propose that CRTC-dependent clock mechanisms have co-evolved with selective clock genes among different species.

Induction of Drosophila Behavioral and Molecular Circadian Rhythms by Temperature Steps in Constant Light

In constant light, where Drosophila rhythms are normally disrupted, temperature cycles induce circadian rhythms at both the molecular and behavioral level. The authors investigated the process by which the thermoperiod induces the rhythms using temperature steps. A 10 °C temperature step-up induced a single locomotor activity peak ca 9 h after the temperature transition, whereas a 10 °C step-down induced a strong activity peak ca 24 h after the transition, and the peak recurred for several cycles, suggesting that the underlying clock is reset. Arrhythmic per01, tim 01, dClkJrk, and cyc01 mutant flies failed to show the rhythm after the step-down, suggesting that per, tim, dClk, and cyc are necessary for the step-down—induced rhythm. After the step-up, per01 flies exhibited an activity peak similar to that of wild-type flies, suggesting that the peak can be induced by the step-up in absence of PER. mRNA levels of per, tim , dClk, vri, and Pdp1ε were changed in response to the temperature steps, but the changes differed depending on the direction of temperature steps, suggesting that steps-up and steps-down have different roles in the initiation of the oscillation. Probably, alternating 12-h temperature steps-up and steps-down will induce opposite changes in mRNA levels of clock genes, eventually producing stable molecular oscillations. Although TIM shows responses to temperature consistent with the changes of its mRNA, this is not the case for PER, consistent with posttranscriptional regulation. Changes of the mRNA levels were significantly altered but still observed in per 01 flies but not observed in dClkJrk flies, except for per mRNA. This suggests that dCLK is involved in the temperature-induced changes in the levels of most clock gene mRNA but that per is regulated via a different mechanism.

Normalizing sleep quality disturbed by psychiatric polypharmacy: a single patient open trial (SPOT)

There is a growing interest in personalized and preventive medicine initiatives that leverage serious patient engagement, such as those initiated and pursued among participants in the quantified-self movement. However, many of the self-assessments that result are not rooted in good scientific practices, such as exploiting controls, dose escalation strategies, multiple endpoint monitoring, etc. Areas where individual monitoring and health assessments have great potential involve sleep and behavior, as there are a number of very problematic sleep and behavior-related conditions that are hard to treat without personalization. For example, winter depression or seasonal affective disorder (SAD) is a serious, recurrent, atypical depressive disorder impacting millions each year. In order to prevent yearly recurrence antidepressant drugs are used to prophylactically treat SAD. In turn, these antidepressant drugs can affect sleep patterns, further exacerbating the condition. Because of this, possibly unique combinatorial or ‘polypharmaceutical’ interventions involving sleep aids may be prescribed. However, little research into the effects of such polypharmacy on the long-term sleep quality of treated individuals has been pursued. Employing wireless monitoring in a patient-centered study we sought to gain insight into the influence of polypharmacy on sleep patterns and the optimal course of therapy for an individual being treated for SAD with duloxetine (Cymbalta) and temazepam. We analyzed continuous-time sleep data while dosages and combinations of these agents were varied. We found that the administration of Cymbalta led to an exacerbation of the subject’s symptoms in a statistically significant way. We argue that such analyses may be necessary to effectively treat individuals with similar overall clinical manifestations and diagnosis, despite their having a unique set of symptoms, genetic profiles and exposure histories. We also consider the limitations of our study and areas for further research.

Temperature-dependent resetting of the molecular circadian oscillator in Drosophila

Circadian clocks responsible for daily time keeping in a wide range of organisms synchronize to daily temperature cycles via pathways that remain poorly understood. To address this problem from the perspective of the molecular oscillator, we monitored temperature-dependent resetting of four of its core components in the fruitfly Drosophila melanogaster: the transcripts and proteins for the clock genes period (per) and timeless (tim). The molecular circadian cycle in adult heads exhibited parallel responses to temperature-mediated resetting at the levels of per transcript, tim transcript and TIM protein. Early phase adjustment specific to per transcript rhythms was explained by clock-independent temperature-driven transcription of per. The cold-induced expression of Drosophila per contrasts with the previously reported heat-induced regulation of mammalian Period 2. An altered and more readily re-entrainable temperature-synchronized circadian oscillator that featured temperature-driven per transcript rhythms and phase-shifted TIM and PER protein rhythms was found for flies of the 'Tim 4' genotype, which lacked daily tim transcript oscillations but maintained post-transcriptional temperature entrainment of tim expression. The accelerated molecular and behavioural temperature entrainment observed for Tim 4 flies indicates that clock-controlled tim expression constrains the rate of temperature cycle-mediated circadian resetting.

Warming Up Your Tick-Tock: Temperature-Dependent Regulation of Circadian Clocks

Circadian clocks are endogenous time-keeping mechanisms to adaptively coordinate animal behaviors and physiology with daily environmental changes. So far many circadian studies in model organisms have identified evolutionarily conserved molecular frames of circadian clock genes in the context of transcription-translation feedback loops. The molecular clockwork drives cell-autonomously cycling gene expression with ~24-hour periodicity, which is fundamental to circadian rhythms. Light and temperature are two of the most potent external time cues to reset the circadian phase of the internal clocks, yet relatively little is known about temperature-relevant clock regulation. In this review, we describe recent findings on temperature-dependent clock mechanisms in homeothermic mammals as compared with poikilothermic Drosophila at molecular, neural, and organismal levels. We propose thermodynamic transitions in RNA secondary structures might have been potent substrates for the molecular evolution of temperature-relevant post-transcriptional mechanisms. Future works should thus validate the potential involvement of specific post-transcriptional steps in temperature-dependent plasticity of circadian clocks.

Integrating circadian activity and gene expression profiles to predict chronotoxicity of Drosophila suzukii response to insecticides

Native to Southeast Asia, Drosophila suzukii (Matsumura) is a recent invader that infests intact ripe and ripening fruit, leading to significant crop losses in the U.S., Canada, and Europe. Since current D. suzukii management strategies rely heavily on insecticide usage and insecticide detoxification gene expression is under circadian regulation in the closely related Drosophila melanogaster, we set out to determine if integrative analysis of daily activity patterns and detoxification gene expression can predict chronotoxicity of D. suzukii to insecticides. Locomotor assays were performed under conditions that approximate a typical summer or winter day in Watsonville, California, where D. suzukii was first detected in North America. As expected, daily activity patterns of D. suzukii appeared quite different between ‘summer’ and ‘winter’ conditions due to differences in photoperiod and temperature. In the ‘summer’, D. suzukii assumed a more bimodal activity pattern, with maximum activity occurring at dawn and dusk. In the ‘winter’, activity was unimodal and restricted to the warmest part of the circadian cycle. Expression analysis of six detoxification genes and acute contact bioassays were performed at multiple circadian times, but only in conditions approximating Watsonville summer, the cropping season, when most insecticide applications occur. Five of the genes tested exhibited rhythmic expression, with the majority showing peak expression at dawn (ZT0, 6am). We observed significant differences in the chronotoxicity of D. suzukii towards malathion, with highest susceptibility at ZT0 (6am), corresponding to peak expression of cytochrome P450s that may be involved in bioactivation of malathion. High activity levels were not found to correlate with high insecticide susceptibility as initially hypothesized. Chronobiology and chronotoxicity of D. suzukii provide valuable insights for monitoring and control efforts, because insect activity as well as insecticide timing and efficacy are crucial considerations for pest management. However, field research is necessary for extrapolation to agricultural settings.

Insect photoperiodic calendar and circadian clock: independence, cooperation, or unity?

The photoperiodic calendar is a seasonal time measurement system which allows insects to cope with annual cycles of environmental conditions. Seasonal timing of entry into diapause is the most often studied photoperiodic response of insects. Research on insect photoperiodism has an approximately 80-year-old tradition. Despite that long history, the physiological mechanisms underlying functionality of the photoperiodic calendar remain poorly understood. Thus far, a consensus has not been reached on the role of another time measurement system, the biological circadian clock, in the photoperiodic calendar. Are the two systems physically separated and functionally independent, or do they cooperate, or is it a single system with dual output? The relationship between calendar and clock functions are the focus of this review, with particular emphasis on the potential roles of circadian clock genes, and the circadian clock system as a whole, in the transduction pathway for photoperiodic token stimulus to the overt expression of facultative diapause.

Molecular genetic analysis of circadian timekeeping in Drosophila

A genetic screen for mutants that alter circadian rhythms in Drosophila identified the first clock gene-the period (per) gene. The per gene is a central player within a transcriptional feedback loop that represents the core mechanism for keeping circadian time in Drosophila and other animals. The per feedback loop, or core loop, is interlocked with the Clock (Clk) feedback loop, but whether the Clk feedback loop contributes to circadian timekeeping is not known. A series of distinct molecular events are thought to control transcriptional feedback in the core loop. The time it takes to complete these events should take much less than 24h, thus delays must be imposed at different steps within the core loop. As new clock genes are identified, the molecular mechanisms responsible for these delays have been revealed in ever-increasing detail and provide an in-depth accounting of how transcriptional feedback loops keep circadian time. The phase of these feedback loops shifts to maintain synchrony with environmental cycles, the most reliable of which is light. Although a great deal is known about cell-autonomous mechanisms of light-induced phase shifting by CRYPTOCHROME (CRY), much less is known about non-cell autonomous mechanisms. CRY mediates phase shifts through an uncharacterized mechanism in certain brain oscillator neurons and carries out a dual role as a photoreceptor and transcription factor in other tissues. Here, I review how transcriptional feedback loops function to keep time in Drosophila, how they impose delays to maintain a 24-h cycle, and how they maintain synchrony with environmental light:dark cycles. The transcriptional feedback loops that keep time in Drosophila are well conserved in other animals, thus what we learn about these loops in Drosophila should continue to provide insight into the operation of analogous transcriptional feedback loops in other animals.

A comparative view of insect circadian clock systems

Recent studies revealed that the neuronal network controlling overt rhythms shows striking similarity in various insect orders. The pigment-dispersing factor seems commonly involved in regulating locomotor activity. However, there are considerable variations in the molecular oscillatory mechanism, and input and output pathways among insects. In Drosophila, autoregulatory negative feedback loops that consist of clock genes, such as period and timeless are believed to create 24-h rhythmicity. Although similar clock genes have been found in some insects, the behavior of their product proteins shows considerable differences from that of Drosophila. In other insects, mammalian-type cryptochrome (cry2) seems to work as a transcriptional repressor in the feedback loop. For photic entrainment, Drosophila type cryptochrome (cry1) plays the major role in Drosophila while the compound eyes are the major photoreceptor in others. Further comparative study will be necessary to understand how this variety of clock mechanisms derived from an ancestral one.

Synergic Entrainment of Drosophila’s Circadian Clock by Light and Temperature

Daily light and temperature cycles are considered the most important zeitgebers for circadian clocks in many organisms. The influence of each single zeitgeber on the clock has been well studied, but little is known about any synergistic effects of both zeitgebers on the clock. In nature, light and temperature show characteristic daily oscillations with the temperature rising during the light phase and reaching its maximum in the late afternoon. Here, we studied behavioral and molecular rhythms in Drosophila melanogaster under simulated natural low light-dark (LD) and temperature (T) cycles that typically occur during the September equinox. Wild-type flies were either subjected to simulated LD or T cycles alone or to a combination of both. Behavioral rhythms and molecular rhythms in the different clock neurons were assessed under the 3 different conditions. Although behavioral rhythms entrained to all conditions, the rhythms were most robust under the combination of LD and T cycles. The clock neurons responded differently to LD and T cycles. Some were not entrained by T cycles alone; others were only slightly entrained by LD cycles alone. The amplitude of the molecular cycling was not different between LD alone and T cycles alone; but LD alone could set the pacemaker neurons to similar phases, whereas T cycles alone could not. The combination of the 2 zeitgebers entrained all clock neurons not only with similar phase but also enhanced the amplitude of Timeless cycling in the majority of cells. Our results show that the 2 zeitgebers synergistically entrain behavioral and molecular rhythms of Drosophila melanogaster.

The right period for a Siesta

Drosophila melanogaster has a broad geographic range. Daily activity in this species exhibits seasonality such that midday rest expands on long warm days, possibly to avoid desiccation. Comparative analyses show that temperature-dependent control of this behavior is partly linked to patterns of per mRNA splicing that are absent in Drosophila yakuba, a related species native to warmer climates with little seasonal change.

Thermosensitive splicing of a clock gene and seasonal adaptation

Similar to many diurnal animals, the daily distribution of activity in Drosophila exhibits a bimodal pattern with clock-controlled morning and evening peaks separated by a midday "siesta." In prior work, we showed that the thermosensitive splicing of a 3'-terminal intron in the RNA product from the Drosophila period (per) gene (dper) is critical for temperature-induced adjustments in the timing of evening activity. Cold temperatures enhance the splicing efficiency of this intron (termed dmpi8, Drosophila melanogaster per intron 8), an event that stimulates the daily accumulation of dper RNA and protein, leading to earlier evening activity. Conversely, warm temperatures attenuate dmpi8 splicing efficiency contributing to delayed evening activity, likely ensuring that flies avoid activity during the hot midday sun when they are at increased risk of desiccation. Here, we discuss the underlying molecular mechanisms governing the thermosensitive splicing of dmpi8 and how it contributes to seasonal changes in the daily activity patterns of Drosophila. On a broader perspective, RNA-RNA interactions likely have fundamental roles in the thermal adaptation of life forms to the daily and seasonal changes in temperature.

Circadian regulation of a limited set of conserved microRNAs in Drosophila

BACKGROUND

MicroRNAs (miRNAs) are short non-coding RNA molecules that target mRNAs to control gene expression by attenuating the translational efficiency and stability of transcripts. They are found in a wide variety of organisms, from plants to insects and humans. Here, we use Drosophila to investigate the possibility that circadian clocks regulate the expression of miRNAs.

RESULTS

We used a microarray platform to survey the daily levels of D. melanogaster miRNAs in adult heads of wildtype flies and the arrhythmic clock mutant cyc01. We find two miRNAs (dme-miR-263a and -263b) that exhibit robust daily changes in abundance in wildtype flies that are abolished in the cyc01 mutant. dme-miR-263a and -263b reach trough levels during the daytime, peak during the night and their levels are constitutively elevated in cyc01 flies. A similar pattern of cycling is also observed in complete darkness, further supporting circadian regulation. In addition, we identified several miRNAs that appear to be constitutively expressed but nevertheless differ in overall daily levels between control and cyc01 flies.

CONCLUSION

The circadian clock regulates miRNA expression in Drosophila, although this appears to be highly restricted to a small number of miRNAs. A common mechanism likely underlies daily changes in the levels of dme-miR-263a and -263b. Our results suggest that cycling miRNAs contribute to daily changes in mRNA and/or protein levels in Drosophila. Intriguingly, the mature forms of dme-miR-263a and -263b are very similar in sequence to several miRNAs recently shown to be under circadian regulation in the mouse retina, suggesting conserved functions.

Separate sets of cerebral clock neurons are responsible for light and temperature entrainment of Drosophila circadian locomotor rhythms

The fruit fly Drosophila melanogaster shows a bimodal circadian locomotor rhythm with peaks at lights-on and before lights-off, which are regulated by multiple clocks in the brain. Even under light-dark cycles, the timing of the evening peak is highly dependent on temperature, starting earlier under lower ambient temperature but terminating almost at the same time. In the present study, using behavioral and immunohistochemical assays, the authors show that separate groups of clock neurons, either light-entrainable or temperature-entrainable, form a functional system driving the locomotor rhythm. When subjected to a light cycle combined with a temperature cycle advanced by 6 h relative to the light cycle, the dorsally located neurons (DNs) and lateral posterior neurons (LPNs) shifted their phase of TIMELESS expression, but the laterally located protocerebral neurons (LNs) basically maintained their original phase. Thus, the LNs seem to be preferentially light-entrainable and the DNs and LPNs to be primarily temperature-entrainable. In pdf(01) mutant flies that lack the neuropeptide PDF in the ventral groups of LNs, the onset of the evening peak was greatly advanced even under synchronized light and temperature cycles and was shifted even more than in wild-type flies in response to a 6-h phase shift of the temperature cycle, suggesting that ventral LNs have a strong impact on the phase of the other cells. It seems likely that the 2 sets of clock cells with different entrainability to light and temperature, and the coupling between them, enable Drosophila to keep a proper phase relationship of circadian activity with respect to the daily light and temperature cycles.

Integration of light and temperature in the regulation of circadian gene expression in Drosophila

Circadian clocks are aligned to the environment via synchronizing signals, or Zeitgebers, such as daily light and temperature cycles, food availability, and social behavior. In this study, we found that genome-wide expression profiles from temperature-entrained flies show a dramatic difference in the presence or absence of a thermocycle. Whereas transcript levels appear to be modified broadly by changes in temperature, there is a specific set of temperature-entrained circadian mRNA profiles that continue to oscillate in constant conditions. There are marked differences in the biological functions represented by temperature-driven or circadian regulation. The set of temperature-entrained circadian transcripts overlaps significantly with a previously defined set of transcripts oscillating in response to a photocycle. In follow-up studies, all thermocycle-entrained circadian transcript rhythms also responded to light/dark entrainment, whereas some photocycle-entrained rhythms did not respond to temperature entrainment. Transcripts encoding the clock components Period, Timeless, Clock, Vrille, PAR-domain protein 1, and Cryptochrome were all confirmed to be rhythmic after entrainment to a daily thermocycle, although the presence of a thermocycle resulted in an unexpected phase difference between period and timeless expression rhythms at the transcript but not the protein level. Generally, transcripts that exhibit circadian rhythms both in response to thermocycles and photocycles maintained the same mutual phase relationships after entrainment by temperature or light. Comparison of the collective temperature- and light-entrained circadian phases of these transcripts indicates that natural environmental light and temperature cycles cooperatively entrain the circadian clock. This interpretation is further supported by comparative analysis of the circadian phases observed for temperature-entrained and light-entrained circadian locomotor behavior. Taken together, these findings suggest that information from both light and temperature is integrated by the transcriptional clock mechanism in the adult fly head.

A key adaptation to life on Earth is provided by internal daily time-keeping mechanisms that allow anticipation of the alternations between night and day. To act as reliable time-keeping mechanisms, circadian clocks have to be able to synchronize to environmental time cues, maintain ∼24-h rhythms under constant conditions, run at approximately the same pace over a range of environmental temperatures, and efficiently communicate time-of-day information to other biological systems. Clock-controlled oscillations in gene expression play an essential role in producing overt circadian rhythms. For most organisms, light/dark cycles appear to constitute the most powerful entrainment cue, but daily temperature cycles have also been demonstrated to efficiently synchronize circadian rhythms. This study uses the fruit fly Drosophila melanogaster as a model to compare the clock-dependent and clock-independent daily gene expression rhythms produced in response to light/dark cycles versus temperature cycles. A broad temperature-driven expression program was found in the heads of both wild-type and arrhythmic mutant flies, but wild-type flies also exhibited a more specific temperature-entrained circadian expression response that resembled the circadian response following light entrainment. The phase relationship between the temperature- and light-entrained circadian rhythms suggests that in nature light and temperature act cooperatively to synchronize the circadian clock.