Extraordinary behavioral entrainment following circadian rhythm bifurcation in mice.

The mammalian circadian timing system uses light to synchronize endogenously generated rhythms with the environmental day. Entrainment to schedules that deviate significantly from 24 h (T24) has been viewed as unlikely because the circadian pacemaker appears capable only of small, incremental responses to brief light exposures. Challenging this view, we demonstrate that simple manipulations of light alone induce extreme plasticity in the circadian system of mice. Firstly, exposure to dim nocturnal illumination (<0.1 lux), rather than completely dark nights, permits expression of an altered circadian waveform wherein mice in light/dark/light/dark (LDLD) cycles "bifurcate" their rhythms into two rest and activity intervals per 24 h. Secondly, this bifurcated state enables mice to adopt stable activity rhythms under 15 or 30 h days (LDLD T15/T30), well beyond conventional limits of entrainment. Continuation of dim light is unnecessary for T15/30 behavioral entrainment following bifurcation. Finally, neither dim light alone nor a shortened night is sufficient for the extraordinary entrainment observed under bifurcation. Thus, we demonstrate in a non-pharmacological, non-genetic manipulation that the circadian system is far more flexible than previously thought. These findings challenge the current conception of entrainment and its underlying principles, and reveal new potential targets for circadian interventions

Environmentally-Induced Entrainment Plasticity: Behavioral Adaptation to Extreme Conditions and its Consequences

The mammalian circadian system is regulated by an internal oscillator that has evolved to keep time in a very predictable rhythmic environment, and is generally considered not flexible enough to adjust to rapidly changing sleep schedules in shiftworkers. This inability to quickly adjust leads to circadian disruption, which is associated with increased risk for chronic disease. Increasing the flexibility of the circadian system could enhance adaptation to irregular cycles, and thereby alleviate negative consequences. This dissertation examines a mouse model for enhanced circadian entrainment, its mechanisms, and its utility for human shiftworkers.Chapter 2 describes that with the addition of dim night time illumination, mice can behaviorally adapt to 18 hour days (T18). T18 entrainment is remarkable and unprecedented in any mammalian system, which typically can only entrain to a narrow range of day-lengths (i.e 22-26 hours). Chapter 3 expands the characterization of this state of enhanced circadian plasticity by describing oscillator characteristics. After demonstrating the lack of circadian oscillator-typical behavior in T18, we conclude that control of behavior in this condition must not derive from entrainment of a conventional circadian oscillator as explained by classical entrainment theories. In Chapter 4, clock gene expression rhythms in the suprachiasmatic nucleus -- a small area in the hypothalamus that orchestrates rhythms in mammals -- and liver and kidney further support the hypothesis that canonical circadian drivers are not involved in control of behavior in T18. While behavior displays 18 hours rhythms, clock gene expression is 24 hours in all tissues. Lastly, Chapter 5 demonstrates that the temporal organization of control of female reproductive function is altered in T18, without any negative impact on reproductive efficacy. Together, this work demonstrates that the rodent circadian system can be markedly more flexible than traditional circadian entrainment theory predicts, but that this flexibility might rely on mechanisms that do not fit with the classical understanding of entrainment systems. Flexible entrainment has translational potential for human shiftworkers to adapt to irregular and sometimes unpredictable work schedules; and does not lead to negative health consequences seen in other non-24h paradigms

Enhanced Circadian Entrainment in Mice and Its Utility under Human Shiftwork Schedules

The circadian system is generally considered to be incapable of adjusting to rapid changes in sleep/work demands. In shiftworkers this leads to chronic circadian disruption and sleep loss, which together predict underperformance at work and negative health consequences. Two distinct experimental protocols have been proposed to increase circadian flexibility in rodents using dim light at night: rhythm bifurcation and T-cycle (i.e., day length) entrainment. Successful translation of such protocols to human shiftworkers could facilitate alignment of internal time with external demands. To assess entrainment flexibility following bifurcation and exposure to T-cycles, mice in Study 1 were repeatedly phase-shifted. Mice from experimental conditions rapidly phase-shifted their activity, while control mice showed expected transient misalignment. In Study 2 and 3, mice followed a several weeks-long intervention designed to model a modified DuPont or Continental shiftwork schedule, respectively. For both schedules, bifurcation and nocturnal dim lighting reduced circadian misalignment. Together, these studies demonstrate proof of concept that mammalian circadian systems can be rendered sufficiently flexible to adapt to multiple, rapidly changing shiftwork schedules. Flexible adaptation to exotic light-dark cycles likely relies on entrainment mechanisms that are distinct from traditional entrainment

Environmentally-Induced Entrainment Plasticity: Behavioral Adaptation to Extreme Conditions and its Consequences

The mammalian circadian system is regulated by an internal oscillator that has evolved to keep time in a very predictable rhythmic environment, and is generally considered not flexible enough to adjust to rapidly changing sleep schedules in shiftworkers. This inability to quickly adjust leads to circadian disruption, which is associated with increased risk for chronic disease. Increasing the flexibility of the circadian system could enhance adaptation to irregular cycles, and thereby alleviate negative consequences. This dissertation examines a mouse model for enhanced circadian entrainment, its mechanisms, and its utility for human shiftworkers.Chapter 2 describes that with the addition of dim night time illumination, mice can behaviorally adapt to 18 hour days (T18). T18 entrainment is remarkable and unprecedented in any mammalian system, which typically can only entrain to a narrow range of day-lengths (i.e 22-26 hours). Chapter 3 expands the characterization of this state of enhanced circadian plasticity by describing oscillator characteristics. After demonstrating the lack of circadian oscillator-typical behavior in T18, we conclude that control of behavior in this condition must not derive from entrainment of a conventional circadian oscillator as explained by classical entrainment theories. In Chapter 4, clock gene expression rhythms in the suprachiasmatic nucleus -- a small area in the hypothalamus that orchestrates rhythms in mammals -- and liver and kidney further support the hypothesis that canonical circadian drivers are not involved in control of behavior in T18. While behavior displays 18 hours rhythms, clock gene expression is 24 hours in all tissues. Lastly, Chapter 5 demonstrates that the temporal organization of control of female reproductive function is altered in T18, without any negative impact on reproductive efficacy. Together, this work demonstrates that the rodent circadian system can be markedly more flexible than traditional circadian entrainment theory predicts, but that this flexibility might rely on mechanisms that do not fit with the classical understanding of entrainment systems. Flexible entrainment has translational potential for human shiftworkers to adapt to irregular and sometimes unpredictable work schedules; and does not lead to negative health consequences seen in other non-24h paradigms

Enhanced Circadian Entrainment in Mice and Its Utility under Human Shiftwork Schedules.

The circadian system is generally considered to be incapable of adjusting to rapid changes in sleep/work demands. In shiftworkers this leads to chronic circadian disruption and sleep loss, which together predict underperformance at work and negative health consequences. Two distinct experimental protocols have been proposed to increase circadian flexibility in rodents using dim light at night: rhythm bifurcation and T-cycle (i.e., day length) entrainment. Successful translation of such protocols to human shiftworkers could facilitate alignment of internal time with external demands. To assess entrainment flexibility following bifurcation and exposure to T-cycles, mice in Study 1 were repeatedly phase-shifted. Mice from experimental conditions rapidly phase-shifted their activity, while control mice showed expected transient misalignment. In Study 2 and 3, mice followed a several weeks-long intervention designed to model a modified DuPont or Continental shiftwork schedule, respectively. For both schedules, bifurcation and nocturnal dim lighting reduced circadian misalignment. Together, these studies demonstrate proof of concept that mammalian circadian systems can be rendered sufficiently flexible to adapt to multiple, rapidly changing shiftwork schedules. Flexible adaptation to exotic light-dark cycles likely relies on entrainment mechanisms that are distinct from traditional entrainment