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Chronotype Variation Drives Night-Time Sentinel-Like Behaviour in Hunter-Gatherers

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Chronotype Variation Drives Night-Time Sentinel-Like Behaviour in Hunter-Gatherers

David R Samson et al. Proc Biol Sci.

Abstract

Sleep is essential for survival, yet it also represents a time of extreme vulnerability to predation, hostile conspecifics and environmental dangers. To reduce the risks of sleeping, the sentinel hypothesis proposes that group-living animals share the task of vigilance during sleep, with some individuals sleeping while others are awake. To investigate sentinel-like behaviour in sleeping humans, we investigated activity patterns at night among Hadza hunter-gatherers of Tanzania. Using actigraphy, we discovered that all subjects were simultaneously scored as asleep for only 18 min in total over 20 days of observation, with a median of eight individuals awake throughout the night-time period; thus, one or more individuals was awake (or in light stages of sleep) during 99.8% of sampled epochs between when the first person went to sleep and the last person awoke. We show that this asynchrony in activity levels is produced by chronotype variation, and that chronotype covaries with age. Thus, asynchronous periods of wakefulness provide an opportunity for vigilance when sleeping in groups. We propose that throughout human evolution, sleeping groups composed of mixed age classes provided a form of vigilance. Chronotype variation and human sleep architecture (including nocturnal awakenings) in modern populations may therefore represent a legacy of natural selection acting in the past to reduce the dangers of sleep.

Keywords: actigraphy; chronotype; evolutionary mismatch; hunter–gatherer; sentinel; sleep.

Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Functional linear modelling (FLM) analysis. Individual 24 h averaged activity (counts per minute) generated from FLM is compared among all individual Hadza subjects throughout the 24 h period. The horizontal red line crosses the y-axis at ‘activity 80.’ This analysis reveals that several individuals exhibited moderate levels of activity throughout the 24 h period in this population, and at no point did the averaged activity of all individuals fit under the conservative threshold of 80 activity counts per minute. In other words, more than one individual was inferred to be active throughout the 24 h period based on this analysis.
Figure 2.
Figure 2.
Circadian phase measure (CPM) in Hadza hunter–gatherers. CPM is the midpoint clock time between sleep onset and awakening. Here, the average CPM is illustrated for each individual with a beanplot, where the horizontal white lines show the individual night-time CPM observations and the black shows the distribution of the observed data; shape of the beanplot represents density. ANOVA revealed that the Hadza significantly differ in CPM on the individual level (see §3c). Additionally, the more intra-individual variation, the greater the spread from the red line; that is, the longer and thinner the bean plots, the more variation is seen on the intra-individual level. Note that the tails of the CPM distributions overlap several night-time hours; in other words, variability in sleep timing is distributed throughout the night-time period.
Figure 3.
Figure 3.
Plotted slope for the fixed coefficient of ‘age.’ Circadian phase measure (CPM) was regressed on age, with circadian phase generated by calculating each individual's nightly number of minutes asleep from after midnight. The model showed that sex, number of co-sleeping children, nursing status and the study day (correcting for weather) did not significantly influence CPM, but age did.
Figure 4.
Figure 4.
Simulating wakefulness dynamics in relation to group size. The model used to assess significance of our results was also used to generally examine predicted mean number of individuals awake (a) and number of epochs in which all individuals were asleep (b), relative to group size. Open circles indicate simulations with no variation in chronotype, while filled circles incorporate variation in chronotype. Thus, variation in chronotype increases substantially the mean number of individuals awake and, for groups of 12 or fewer, reduces the duration of time when all individuals are asleep.

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