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Comparative Study
. 2008 Jul;62(7):1764-1776.
doi: 10.1111/j.1558-5646.2008.00392.x.

Phylogenetic analysis of the ecology and evolution of mammalian sleep

Affiliations
Comparative Study

Phylogenetic analysis of the ecology and evolution of mammalian sleep

Isabella Capellini et al. Evolution. 2008 Jul.

Abstract

The amount of time asleep varies greatly in mammals, from 3 h in the donkey to 20 h in the armadillo. Previous comparative studies have suggested several functional explanations for interspecific variation in both the total time spent asleep and in rapid-eye movement (REM) or "quiet" (non-REM) sleep. In support of specific functional benefits of sleep, these studies reported correlations between time in specific sleep states (NREM or REM) and brain size, metabolic rate, and developmental variables. Here we show that estimates of sleep duration are significantly influenced by the laboratory conditions under which data are collected and that, when analyses are limited to data collected under more standardized procedures, traditional functional explanations for interspecific variation in sleep durations are no longer supported. Specifically, we find that basal metabolic rate correlates negatively rather than positively with sleep quotas, and that neither adult nor neonatal brain mass correlates positively with REM or NREM sleep times. These results contradict hypotheses that invoke energy conservation, cognition, and development as drivers of sleep variation. Instead, the negative correlations of both sleep states with basal metabolic rate and diet are consistent with trade-offs between sleep and foraging time. In terms of predation risk, both REM and NREM sleep quotas are reduced when animals sleep in more exposed sites, whereas species that sleep socially sleep less. Together with the fact that REM and NREM sleep quotas correlate strongly with each other, these results suggest that variation in sleep primarily reflects ecological constraints acting on total sleep time, rather than the independent responses of each sleep state to specific selection pressures. We propose that, within this ecological framework, interspecific variation in sleep duration might be compensated by variation in the physiological intensity of sleep.

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Figures

Figure 1
Figure 1
Laboratory conditions: (A) Total daily sleep time (TST), REM, and NREM sleep quotas in studies with < 12 h recording time (group 1), between 12 and < 24 recording time (including 12 h; group 2), and in studies with ≥ 24 h recording time (group 3; see text). (B) TST as estimated by EEG and nonEEG studies. Boxes show upper and lower quartiles with the horizontal line indicating the median, whiskers outside the box delimit are drawn at 1.5 interquartile range, and circles represent outliers.
Figure 2
Figure 2
Bivariate relationships between phylogenetically independent contrasts of NREM and REM sleep quotas in mammals.
Figure 3
Figure 3
Bivariate relationships between phylogenetically independent contrasts in NREM (A) and REM (B) with contrasts in relative BMR.
Figure 4
Figure 4
Bivariate relationship between contrasts in NREM (A) and REM (B) with relative gestation length.
Figure 5
Figure 5
Bivariate relationship of contrasts in NREM and REM with relative sleep site exposure (A and B), social sleep behavior (C and D), and diet (E and F).

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