Abstract
Study ObjectivesIt has long been suspected that sleep is important for regulating body temperature and metabolic-rate. Hibernation, a state of acute hypothermia and reduced metabolic-rate, offers a promising system for investigating those relationships. Prior studies in hibernating ground squirrels report that, although sleep occurs during hibernation, it manifests only as non-REM sleep, and only at relatively high temperatures. In our study, we report data on sleep during hibernation in a lemuriform primate, Cheirogaleus medius. As the only primate known to experience prolonged periods of hibernation and as an inhabitant of more temperate climates than ground squirrels, this animal serves as an alternative model for exploring sleep temperature/metabolism relationships that may be uniquely relevant to understanding human physiology.Measurements and ResultsWe find that during hibernation, non-REM sleep is absent in Cheirogaleus. Rather, periods of REM sleep occur during periods of relatively high ambient temperature, a pattern opposite of that observed in ground squirrels. Like ground squirrels, however, EEG is marked by ultra-low voltage activity at relatively low metabolic-rates.ConclusionsThese findings confirm a sleep-temperature/metabolism link, though they also suggest that the relationship of sleep stage with temperature/metabolism is flexible and may differ across species or mammalian orders. The absence of non-REM sleep suggests that during hibernation in Cheirogaleus, like in the ground squirrel, the otherwise universal non-REM sleep homeostatic response is greatly curtailed or absent. Lastly, ultra-low voltage EEG appears to be a cross-species marker for extremely low metabolic-rate, and, as such, may be an attractive target for research on hibernation induction.
Highlights
The specific functions of sleep remain unknown
[20] EEG slow-wave activity recorded during sleep during arousals occurs in a pattern that is consistent with the hypothesis that ground squirrels are sleep deprived when they emerge from torpor bouts in proportion to the length of the torpor bout and in inverse proportion to the time slept during hibernation. [18,19,20,21,22,23] several studies which prevented sleep from occurring during arousals from torpor bouts demonstrate that this EEG slow-wave activity is not, a manifestation of sleep deprivation following periods of torpor. [24,25] the purpose of the period arousals remains unknown and there is evidence that torpor eliminates or decreases the usual need for sleep as manifested in the homeostatic build-up of sleep drive that develops when sleep does not occur over time
The EEG data manifested the classical features of mammalian sleep comprised of non-rapid eye movement (NREM) sleep associated with an increase in low-frequency EEG activity and REM sleep marked by decreased EMG activity and rapid eyemovements (See Figure 1)
Summary
It has been hypothesized that sleep may play a role in the regulation of temperature and metabolism based on several lines of research suggesting that these phenomena are highly inter-related. [13] Further, in fruit flies and mammals, waking and short-term sleep deprivation up-regulate metabolismrelated genes [14,15] Based on such observations, it has been hypothesized that the regulation of temperature and metabolism are intimately related to the ultimate functions of sleep and that the study of sleep during hibernation, a state where metabolic rate dramatically decreases and core body temperature drifts towards ambient, is of particular interest for elucidating these relationships. In ground squirrels REM sleep appears to be absent during hibernation and non-REM sleep is more likely at higher temperatures, such that continuous non-REM sleep is seen during hibernation at moderate temperature. [18,19] Periodic arousals to euthermia are reported in ground squirrels hibernating at low temperature where sleep does not occur and some have hypothesized that this occurs to allow sleep which is a necessity. [20] EEG slow-wave activity recorded during sleep during arousals occurs in a pattern that is consistent with the hypothesis that ground squirrels are sleep deprived when they emerge from torpor bouts in proportion to the length of the torpor bout and in inverse proportion to the time slept during hibernation. [18,19,20,21,22,23] several studies which prevented sleep from occurring during arousals from torpor bouts demonstrate that this EEG slow-wave activity is not, a manifestation of sleep deprivation following periods of torpor. [24,25] the purpose of the period arousals remains unknown and there is evidence that torpor eliminates or decreases the usual need for sleep as manifested in the homeostatic build-up of sleep drive that develops when sleep does not occur over time
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