BACKGROUND
the critical role of the suprachiasmatic nucleus  in the orchestration of circadian rhythms represents one of the most successful brain-behavior relationships established in mammalian neurobiology  <cit> .
the cellular and molecular bases of circadian rhythmicity are similarly well characterized  <cit> .
despite these advances, the practical manipulation of human rhythms remains unrealized.
most shift-workers, for instance, fail to reset their circadian pacemakers to effect an alignment of the work shift with the worker's circadian day  <cit> .
this misalignment may be a contributing factor in the higher nighttime incidence of industrial accidents and the compromised health of shift-workers  <cit> .
in the laboratory under free-running conditions, humans and other mammals typically undergo the rhythmic alternation between physiological and behavioral states appropriate for day  and for night   <cit> .
although individual rhythmic outputs  might be observed to be bimodally expressed, more proximate measures of clock function  tend to alternate unimodally with a period near  <dig> h  <cit> .
one approach to shift-work involves rapid phase-shifting of the entire circadian program to an optimal compromise between the conflicting demands of work and home schedules  <cit> .
a re-entrainment strategy that manipulates only circadian phase, however, can be easily undone by the entraining effects of bright light exposure in transit to and from work and by exposure to sunlight during the day, both of which serve to reinforce normal diurnal entrainment  <cit> .
additionally, even effective re-entrainment of circadian phase may have costs in terms of the resulting coincidence of subjective night with social demands during the day.
an alternative solution to the problem of shift-work might involve focusing on change in circadian waveform instead of phase.
under appropriate lighting conditions, hamsters and mice readily adopt stable, multi-modal entrainment patterns that may meet the requirements of shift-workers for separate intervals of alertness during both the night and the day  <cit> .
these entrainment patterns, furthermore, do not require the complete avoidance of daytime exposure to sunlight.
specifically, rodents maintained in  <dig> h light:dark:light:dark cycles  may entrain their circadian pacemakers so that nocturnal locomotor activity is programmed during each of the two daily scotophases  <dig> h apart .
a series of behavioral and physiological studies has indicated that this reorganization reflects a global splitting of the circadian pacemaker into two oscillatory components that cycle out of phase  <cit>  .
the net result is that, in each  <dig> h period, the subject goes through two short subjective nights interrupted by two short subjective days.
this entrainable split activity pattern occurs in each of three rodent species examined, although the stimulus requirements for maintaining stable entrainment apparently differ by species  <cit> .
to date, two factors have been shown to influence the incidence of rhythm splitting in ldld cycles.
first, animals that engage in robust wheel-running in response to scheduled daily exercise are more likely to split their activity patterns than are animals that remain inactive  <cit> , and splitting often coincides with a cage change that provokes wheel-running activity  <cit> .
second, the presence of dim illumination  during all dark periods markedly increases the proportion of animals that splits in ldld  <cit> .
these two effects may be related as animals are sometimes more active in dim light than in complete darkness, although this cannot wholly account for the facilitative effects of dim light on splitting  <cit> .
in the present experiments, we assessed directly whether the feedback from a running wheel was necessary for the induction and maintenance of rhythm splitting.
additionally, we examined how split rhythms changed as a function of the relative length of day and night.
identification of factors that influence rhythm splitting should serve to clarify underlying mechanisms and inform the design of protocols examining the feasibility of similar entrainment in humans.
finally, body temperature  telemetry allowed assessment of circadian markers that may be mediated by mechanisms separate from those underlying locomotor activity  <cit> .
RESULTS
experiment # <dig>  is a wheel necessary for maintenance and induction of splitting in syrian hamsters?
transfer of syrian hamsters from standard lighting conditions  to ldld7:5:7: <dig> and subsequently to ldld9:3:9: <dig> yielded the patterns of circadian re-entrainment depicted in figure  <dig>  in the first two weeks of ldld7:5:7: <dig>  elevations of general locomotor  activity and tb reliably occurred during each of two daily scotophases for several animals .
in some cases, these rhythms permanently  or transiently  fused into a unimodal  pattern following immobilization of the running wheel.
other hamsters adopted bimodal tb and gl rhythms in ldld7:5:7: <dig> only after immobilization of the running wheel .
still other hamsters first exhibited split rhythms under ldld9:3:9: <dig>  in all cases, split rhythms fused into the unsplit pattern under constant conditions .
table  <dig> indicates the number of animals meeting objective criteria for split and unsplit rhythms in each of the experimental intervals.
there were no discrepancies between the two measures.
because of limited battery life of the telemeters, the number of reported animals decreased as the experiment progressed.
is the tb rhythm distinct from that of activity?
daily mean tb and amplitude of the circadian tb rhythm varied across the experimental intervals .
mean tb, but not tb amplitude, increased following initial transfer to running wheels and ldld7:5:7: <dig> .
immobilization of wheels  <dig> weeks later reduced tb amplitude , whereas the reduction in mean tb just failed to meet statistical significance after correction for multiple comparisons .
in no interval did mean tb or tb amplitude differ for split versus unsplit subjects .
for each individual animal, tb was positively correlated with levels of general activity .
because of likely masking effects of activity on tb, we assessed whether there existed an endogenous tb rhythm independent of locomotor activity using linear regression to partial out the influence of gl on tb rhythms.
the residual tb not accounted for by gl activity exhibited a reliable daily rhythm in individual-and group-analyzed hamsters during exposure to ldld9:3:9: <dig> .
among split hamsters, there were two unambiguous and equal-sized peaks and troughs daily in the endogenous tb rhythm .
for unsplit hamsters, there was a substantial rise only during the subjective night and much smaller elevation in antiphase .
experiment #2: do siberian hamsters require wheels for splitting?
the transfer of siberian hamsters from ld14: <dig> to ldld7:5:7: <dig> and subsequently to ldld cycles with progressively shorter scotophases  induced in individual animals the same split and unsplit rhythms described in experiment # <dig> .
a majority of animals with wheels demonstrated split activity bouts for at least one photoperiod condition, but not before ldld8:4:8: <dig> .
animals lacking running wheels and instead monitored by passive infrared  motion detectors did not differ statistically in the incidence of splitting as compared to the wheel group .
the number of animals demonstrating split activity tended to increase or remain the same as the length of the scotophases was shortened with the exception of pir animals in ldld11:1:11: <dig>  split rhythms were restored upon return to ldld9:3:9: <dig>  however.
of the animals that displayed split rhythms during the second exposure to ldld 9:3:9: <dig> ,  <dig> maintained a split-activity rhythm when transferred to "skeleton photoperiods" .
activity of four of these split animals "phase-jumped" into the two longer  of the four daily scotophases , and one animal that was previously unsplit met the criterion for splitting during the skeleton photoperiod.
the activity duration  of each activity component of split animals varied with the length of the scotophase regardless of whether hamsters had wheels or not .
phase angle of entrainment for the nighttime, but not the daytime component, was positively correlated with scotophase duration for animals with wheels .
this measure was negatively correlated with scotophase duration in animals lacking wheels .
for unsplit hamsters, the expected negative correlation between phase angle of entrainment and scotophase duration was obtained only in animals monitored with motion detectors .
discussion
as reported in previous studies, a substantial fraction of each of two hamster species readily adopts bimodal wheel-running rhythms under  <dig> h ldld cycles  <cit> .
three aspects of the present studies validate the use of measures besides wheel-running to identify these split circadian entrainment patterns.
in experiment # <dig>  the simultaneous recording of tb, general locomotor activity and wheel-running yielded essentially identical rhythms within individual animals.
second, separate groups of siberian hamsters in experiment # <dig> monitored by pir motion detectors and by wheels generated very similar constellations of activity patterns .
finally, the entrainment patterns identified here with motion detectors also mirror wheel-running patterns reported in other studies  <cit> .
the persistence of the split entrainment state in some hamsters exposed to skeleton photoperiods is particularly informative as it discounts the possibility that split activity patterns derive from a) a single long subjective night rendered bimodal by negative masking by light or b) a single short subjective night plus a second non-circadian component positively masked by the second dark period.
the formal basis of this split pattern has been considered in detail elsewhere  <cit> .
the use of measures other than wheel running to identify split rhythms permits testing of the hypothesis that a running wheel is necessary to maintain and/or induce this entrainment pattern.
in previous studies, animals that failed to run in a wheel during scheduled daily exposure to novel wheels also failed to later exhibit split home-cage running patterns  <cit> .
additionally, among animals that did not split immediately in ldld7:5:7: <dig>  we have repeatedly observed abrupt splitting following a cage change during the daytime dark phase  <cit> , which typically induces robust wheel-running activity.
the present results clearly establish that a wheel is not necessary for splitting in either species: in experiment # <dig>  split rhythms in ldld persisted in several syrian hamsters following withdrawal of wheel access, and other hamsters later adopted split rhythms in the absence of a functional wheel when the length of each of the daily photophases was lengthened.
wheel immobilization was not necessarily responsible for the loss of splitting in some hamsters  because rejoining has been observed in the first weeks of chronic wheel access in similar experiments  <cit> .
in experiment # <dig>  siberian hamsters exhibited split rhythms with or without access to a wheel at rates that did not differ between groups.
thus, wheels are necessary neither for maintenance nor for induction of split rhythms.
these experiments, however, were not designed with sufficient statistical power to rule out a minor influence of a running wheel on split rhythms.
whereas it was expected that general locomotion and wheel-running – both measures of activity – would correspond closely, we hypothesized that the tb rhythm might differ from activity rhythms if it was controlled by a distinct oscillatory mechanism as suggested in several, but not all, studies  <cit> .
as noted above, tb rhythms were split in all animals with split activity patterns.
because wheel running likely elevates tb  <cit> , this bimodal pattern might be expected on the basis of masking alone, and indeed, in this study tb rhythms were acutely influenced by access to running wheels.
the wheel-running masking account can be dismissed, however, by the persistence of split tb patterns after the wheels were immobilized as has been reported with constant light-induced splitting  <cit> .
highly significant correlations between tb and gl values raise the vexing possibility that general locomotor activity, as well as wheel running, can have masking effects  <cit> .
adjustment of the tb data to control for the correlation between tb and gl diminished the amplitude but did not eliminate the rhythmicity of the endogenous tb rhythm, suggesting that an activity-independent tb rhythm is split in the same fashion as that of activity.
of course, the estimate of the activity-independent tb rhythm is only as good as the statistical model relating tb and gl.
use of a number of different statistical models , did not alter our basic conclusion.
as illustrated in fig.
<dig> and in previous studies  <cit> , splitting of syrian hamster rhythms under ldld7:5:7: <dig> is not always stable nor was activity always equally distributed between scotophases.
in experiment # <dig>  an increase in splitting incidence was noted after transfer to ldld9:3:9: <dig>  and no split animal ever rejoined under this photoperiod suggesting that this lighting condition may promote stable splitting.
this finding accords with other of our unpublished data from wheel-running hamsters.
the same trend was apparent in siberian hamsters of experiment # <dig> where splitting was numerically most common under nights shorter than  <dig> h. under the shortest nights, however, split rhythms largely disappeared and were later recovered in ldld9:3:9: <dig>  these observations are consistent with a model of splitting in which compression of subjective night reduces the stability of the unsplit pacemaker  <cit> , and thus a split anti-phase entrainment pattern is preferred.
the phase-jumping of split rhythms of several siberian hamsters in skeleton photoperiods further suggests that the individual split components are not fully stable under  <dig> h scotophases.
further work will be necessary to determine whether a circadian parameter related to "stability" can be operationalized and convincingly distinguished from known parameters such as period, phase and phase response to light.
the multiple oscillations that underlie splitting under ldld do not share the same physiological basis as those observed after prolonged exposure to constant light  <cit> .
whereas the latter case is associated with anti-phase oscillations of the left and right scn, the former appears to exhibit no such laterality  <cit> .
whether the two ldld oscillations correspond to the as yet unidentified dual oscillators proposed to underlie photoperiodism, however, remains an open question.
at a formal level of analysis, distinct "evening" and "morning" oscillators nicely account for the photoperiodic regulation of activity duration  of unsplit animals  <cit> .
because similar photoperiodic regulation of α is seen for each component of the split rhythms in experiment # <dig>  each of the two split components may be similarly generated by a complex pacemaker comprising multiple oscillators.
supporting this inference, the relationship between phase angle of entrainment and photoperiod for pir data in experiment # <dig> parallels that of unsplit pacemakers  <cit> , albeit non-significantly in the case of the daytime component.
thus, the circadian system likely comprises not just two functional oscillators, but many, which may couple to generate a variety of entrainment states.
because all mammalian circadian pacemakers likely derive from multiple oscillators in the scn, we suspect that this split entrainment pattern could be induced in other species under permissive conditions.
the practicality and benefit of this entrainment regime to human subjects await evaluation.
at least superficially, it may better meet the needs of shift-workers than present phase-shift protocols that attempt to schedule sleep during the real day without achieving full-scale phase shifts of subjective night.
with a split circadian rhythm, shift-workers might schedule sleep in two equal intervals sandwiched between a late shift and daytime activity.
if the human and hamster pacemakers respond similarly, bright light exposure before and after each sleep phase would reinforce the split pattern rather than compromise entrainment as is the case in some phase-shift protocols.
it also remains to be determined whether this circadian arrangement entails any physiological costs, and if so, whether these outweigh potential benefits.
from a translational perspective, we are encouraged that split rhythms can be observed under a broader range of conditions than initially identified.
human splitting protocols should consider the influence of factors identified in animal studies: nocturnal illumination, photoperiod and activity levels.
future animal work will assess the influence of age, endocrine status and photoperiodic history.
CONCLUSIONS
entrainment of novel circadian waveforms is possible with the use of exotic lighting conditions and may represent a productive new strategy for chronotherapeutics.
while prior studies point to a role of locomotor activity in inducing split activity patterns, access to running wheels is not necessary to induce or to maintain these entrainment patterns.
instead, split entrainment is influenced by the relative length of light and dark phases of the  <dig> h ldld cycle.

 the critical role of the suprachiasmatic nucleus in the orchestration of circadian rhythms represents one of the most successful brain-behavior relationships established in mammalian neurobiology. the cellular and molecular bases of circadian rhythmicity are similarly well characterized. despite these advances, the practical manipulation of human rhythms remains unrealized. most shift-workers, for instance, fail to reset their circadian pacemakers to effect an alignment of the work shift with the worker's circadian day. this misalignment may be a contributing factor in the higher nighttime incidence of industrial accidents and the compromised health of shift-workers. in the laboratory under free-running conditions, humans and other mammals typically undergo the rhythmic alternation between physiological and behavioral states appropriate for day and for night. although individual rhythmic outputs might be observed to be bimodally expressed, more proximate measures of clock function tend to alternate unimodally with a period near h. one approach to shift-work involves rapid phase-shifting of the entire circadian program to an optimal compromise between the conflicting demands of work and home schedules. a re-entrainment strategy that manipulates only circadian phase, however, can be easily undone by the entraining effects of bright light exposure in transit to and from work and by exposure to sunlight during the day, both of which serve to reinforce normal diurnal entrainment. additionally, even effective re-entrainment of circadian phase may have costs in terms of the resulting coincidence of subjective night with social demands during the day. an alternative solution to the problem of shift-work might involve focusing on change in circadian waveform instead of phase. under appropriate lighting conditions, hamsters and mice readily adopt stable, multi-modal entrainment patterns that may meet the requirements of shift-workers for separate intervals of alertness during both the night and the day. these entrainment patterns, furthermore, do not require the complete avoidance of daytime exposure to sunlight. specifically, rodents maintained in h light:dark:light:dark cycles may entrain their circadian pacemakers so that nocturnal locomotor activity is programmed during each of the two daily scotophases h apart. a series of behavioral and physiological studies has indicated that this reorganization reflects a global splitting of the circadian pacemaker into two oscillatory components that cycle out of phase. the net result is that, in each h period, the subject goes through two short subjective nights interrupted by two short subjective days. this entrainable split activity pattern occurs in each of three rodent species examined, although the stimulus requirements for maintaining stable entrainment apparently differ by species. to date, two factors have been shown to influence the incidence of rhythm splitting in ldld cycles. first, animals that engage in robust wheel-running in response to scheduled daily exercise are more likely to split their activity patterns than are animals that remain inactive, and splitting often coincides with a cage change that provokes wheel-running activity. second, the presence of dim illumination during all dark periods markedly increases the proportion of animals that splits in ldld. these two effects may be related as animals are sometimes more active in dim light than in complete darkness, although this cannot wholly account for the facilitative effects of dim light on splitting. in the present experiments, we assessed directly whether the feedback from a running wheel was necessary for the induction and maintenance of rhythm splitting. additionally, we examined how split rhythms changed as a function of the relative length of day and night. identification of factors that influence rhythm splitting should serve to clarify underlying mechanisms and inform the design of protocols examining the feasibility of similar entrainment in humans. finally, body temperature telemetry allowed assessment of circadian markers that may be mediated by mechanisms separate from those underlying locomotor activity.  experiment # is a wheel necessary for maintenance and induction of splitting in syrian hamsters? transfer of syrian hamsters from standard lighting conditions to ldld7:5:7: and subsequently to ldld9:3:9: yielded the patterns of circadian re-entrainment depicted in figure in the first two weeks of ldld7:5:7: elevations of general locomotor activity and tb reliably occurred during each of two daily scotophases for several animals. in some cases, these rhythms permanently or transiently fused into a unimodal pattern following immobilization of the running wheel. other hamsters adopted bimodal tb and gl rhythms in ldld7:5:7: only after immobilization of the running wheel. still other hamsters first exhibited split rhythms under ldld9:3:9: in all cases, split rhythms fused into the unsplit pattern under constant conditions. table indicates the number of animals meeting objective criteria for split and unsplit rhythms in each of the experimental intervals. there were no discrepancies between the two measures. because of limited battery life of the telemeters, the number of reported animals decreased as the experiment progressed. is the tb rhythm distinct from that of activity? daily mean tb and amplitude of the circadian tb rhythm varied across the experimental intervals. mean tb, but not tb amplitude, increased following initial transfer to running wheels and ldld7:5:7:. immobilization of wheels weeks later reduced tb amplitude, whereas the reduction in mean tb just failed to meet statistical significance after correction for multiple comparisons. in no interval did mean tb or tb amplitude differ for split versus unsplit subjects. for each individual animal, tb was positively correlated with levels of general activity. because of likely masking effects of activity on tb, we assessed whether there existed an endogenous tb rhythm independent of locomotor activity using linear regression to partial out the influence of gl on tb rhythms. the residual tb not accounted for by gl activity exhibited a reliable daily rhythm in individual-and group-analyzed hamsters during exposure to ldld9:3:9:. among split hamsters, there were two unambiguous and equal-sized peaks and troughs daily in the endogenous tb rhythm. for unsplit hamsters, there was a substantial rise only during the subjective night and much smaller elevation in antiphase. experiment #2: do siberian hamsters require wheels for splitting? the transfer of siberian hamsters from ld14: to ldld7:5:7: and subsequently to ldld cycles with progressively shorter scotophases induced in individual animals the same split and unsplit rhythms described in experiment #. a majority of animals with wheels demonstrated split activity bouts for at least one photoperiod condition, but not before ldld8:4:8:. animals lacking running wheels and instead monitored by passive infrared motion detectors did not differ statistically in the incidence of splitting as compared to the wheel group. the number of animals demonstrating split activity tended to increase or remain the same as the length of the scotophases was shortened with the exception of pir animals in ldld11:1:11: split rhythms were restored upon return to ldld9:3:9: however. of the animals that displayed split rhythms during the second exposure to ldld 9:3:9:, maintained a split-activity rhythm when transferred to "skeleton photoperiods". activity of four of these split animals "phase-jumped" into the two longer of the four daily scotophases, and one animal that was previously unsplit met the criterion for splitting during the skeleton photoperiod. the activity duration of each activity component of split animals varied with the length of the scotophase regardless of whether hamsters had wheels or not. phase angle of entrainment for the nighttime, but not the daytime component, was positively correlated with scotophase duration for animals with wheels. this measure was negatively correlated with scotophase duration in animals lacking wheels. for unsplit hamsters, the expected negative correlation between phase angle of entrainment and scotophase duration was obtained only in animals monitored with motion detectors. discussion as reported in previous studies, a substantial fraction of each of two hamster species readily adopts bimodal wheel-running rhythms under h ldld cycles. three aspects of the present studies validate the use of measures besides wheel-running to identify these split circadian entrainment patterns. in experiment # the simultaneous recording of tb, general locomotor activity and wheel-running yielded essentially identical rhythms within individual animals. second, separate groups of siberian hamsters in experiment # monitored by pir motion detectors and by wheels generated very similar constellations of activity patterns. finally, the entrainment patterns identified here with motion detectors also mirror wheel-running patterns reported in other studies. the persistence of the split entrainment state in some hamsters exposed to skeleton photoperiods is particularly informative as it discounts the possibility that split activity patterns derive from a) a single long subjective night rendered bimodal by negative masking by light or b) a single short subjective night plus a second non-circadian component positively masked by the second dark period. the formal basis of this split pattern has been considered in detail elsewhere. the use of measures other than wheel running to identify split rhythms permits testing of the hypothesis that a running wheel is necessary to maintain and/or induce this entrainment pattern. in previous studies, animals that failed to run in a wheel during scheduled daily exposure to novel wheels also failed to later exhibit split home-cage running patterns. additionally, among animals that did not split immediately in ldld7:5:7: we have repeatedly observed abrupt splitting following a cage change during the daytime dark phase, which typically induces robust wheel-running activity. the present results clearly establish that a wheel is not necessary for splitting in either species: in experiment # split rhythms in ldld persisted in several syrian hamsters following withdrawal of wheel access, and other hamsters later adopted split rhythms in the absence of a functional wheel when the length of each of the daily photophases was lengthened. wheel immobilization was not necessarily responsible for the loss of splitting in some hamsters because rejoining has been observed in the first weeks of chronic wheel access in similar experiments. in experiment # siberian hamsters exhibited split rhythms with or without access to a wheel at rates that did not differ between groups. thus, wheels are necessary neither for maintenance nor for induction of split rhythms. these experiments, however, were not designed with sufficient statistical power to rule out a minor influence of a running wheel on split rhythms. whereas it was expected that general locomotion and wheel-running – both measures of activity – would correspond closely, we hypothesized that the tb rhythm might differ from activity rhythms if it was controlled by a distinct oscillatory mechanism as suggested in several, but not all, studies. as noted above, tb rhythms were split in all animals with split activity patterns. because wheel running likely elevates tb, this bimodal pattern might be expected on the basis of masking alone, and indeed, in this study tb rhythms were acutely influenced by access to running wheels. the wheel-running masking account can be dismissed, however, by the persistence of split tb patterns after the wheels were immobilized as has been reported with constant light-induced splitting. highly significant correlations between tb and gl values raise the vexing possibility that general locomotor activity, as well as wheel running, can have masking effects. adjustment of the tb data to control for the correlation between tb and gl diminished the amplitude but did not eliminate the rhythmicity of the endogenous tb rhythm, suggesting that an activity-independent tb rhythm is split in the same fashion as that of activity. of course, the estimate of the activity-independent tb rhythm is only as good as the statistical model relating tb and gl. use of a number of different statistical models, did not alter our basic conclusion. as illustrated in fig. and in previous studies, splitting of syrian hamster rhythms under ldld7:5:7: is not always stable nor was activity always equally distributed between scotophases. in experiment # an increase in splitting incidence was noted after transfer to ldld9:3:9: and no split animal ever rejoined under this photoperiod suggesting that this lighting condition may promote stable splitting. this finding accords with other of our unpublished data from wheel-running hamsters. the same trend was apparent in siberian hamsters of experiment # where splitting was numerically most common under nights shorter than h. under the shortest nights, however, split rhythms largely disappeared and were later recovered in ldld9:3:9: these observations are consistent with a model of splitting in which compression of subjective night reduces the stability of the unsplit pacemaker, and thus a split anti-phase entrainment pattern is preferred. the phase-jumping of split rhythms of several siberian hamsters in skeleton photoperiods further suggests that the individual split components are not fully stable under h scotophases. further work will be necessary to determine whether a circadian parameter related to "stability" can be operationalized and convincingly distinguished from known parameters such as period, phase and phase response to light. the multiple oscillations that underlie splitting under ldld do not share the same physiological basis as those observed after prolonged exposure to constant light. whereas the latter case is associated with anti-phase oscillations of the left and right scn, the former appears to exhibit no such laterality. whether the two ldld oscillations correspond to the as yet unidentified dual oscillators proposed to underlie photoperiodism, however, remains an open question. at a formal level of analysis, distinct "evening" and "morning" oscillators nicely account for the photoperiodic regulation of activity duration of unsplit animals. because similar photoperiodic regulation of α is seen for each component of the split rhythms in experiment # each of the two split components may be similarly generated by a complex pacemaker comprising multiple oscillators. supporting this inference, the relationship between phase angle of entrainment and photoperiod for pir data in experiment # parallels that of unsplit pacemakers, albeit non-significantly in the case of the daytime component. thus, the circadian system likely comprises not just two functional oscillators, but many, which may couple to generate a variety of entrainment states. because all mammalian circadian pacemakers likely derive from multiple oscillators in the scn, we suspect that this split entrainment pattern could be induced in other species under permissive conditions. the practicality and benefit of this entrainment regime to human subjects await evaluation. at least superficially, it may better meet the needs of shift-workers than present phase-shift protocols that attempt to schedule sleep during the real day without achieving full-scale phase shifts of subjective night. with a split circadian rhythm, shift-workers might schedule sleep in two equal intervals sandwiched between a late shift and daytime activity. if the human and hamster pacemakers respond similarly, bright light exposure before and after each sleep phase would reinforce the split pattern rather than compromise entrainment as is the case in some phase-shift protocols. it also remains to be determined whether this circadian arrangement entails any physiological costs, and if so, whether these outweigh potential benefits. from a translational perspective, we are encouraged that split rhythms can be observed under a broader range of conditions than initially identified. human splitting protocols should consider the influence of factors identified in animal studies: nocturnal illumination, photoperiod and activity levels. future animal work will assess the influence of age, endocrine status and photoperiodic history.  entrainment of novel circadian waveforms is possible with the use of exotic lighting conditions and may represent a productive new strategy for chronotherapeutics. while prior studies point to a role of locomotor activity in inducing split activity patterns, access to running wheels is not necessary to induce or to maintain these entrainment patterns. instead, split entrainment is influenced by the relative length of light and dark phases of the h ldld cycle. 
we examine here how the relative length of the light and dark phases of the environmental cycle influences this rhythm splitting and the necessity of a running wheel for expression of this entrainment condition. these circadian entrainment patterns may be of therapeutic value to human shift-workers and others facing chronobiological challenges. access to a running wheel was necessary neither for the induction nor maintenance of this entrainment pattern. rhythm splitting was observed in wheel-running and general locomotion of siberian and syrian hamsters.
 in the laboratory, behavioral and physiological states of nocturnal rodents alternate, with a period near h, between those appropriate for the night and for the day. under appropriate h light:dark:light:dark conditions, however, rodents may be readily induced to express bimodal rest/activity cycles that reflect a global temporal reorganization of the central neural pacemaker in the hypothalamus. we examine here how the relative length of the light and dark phases of the environmental cycle influences this rhythm splitting and the necessity of a running wheel for expression of this entrainment condition.  rhythm splitting was observed in wheel-running and general locomotion of siberian and syrian hamsters. the latter also manifest split rhythms in body temperature. access to a running wheel was necessary neither for the induction nor maintenance of this entrainment pattern. while rhythms were only transiently split in many animals with two h nights, the incidence of splitting was greater with twice daily nights of shorter duration. removal of running wheels altered the body temperature rhythm but did not eliminate its clear bimodality.  the expression of entrained, split circadian rhythms exhibits no strict dependence on access to a running wheel, but can be facilitated by manipulation of ambient lighting conditions. these circadian entrainment patterns may be of therapeutic value to human shift-workers and others facing chronobiological challenges. 