Chronotype variation drives night-time sentinel-like behaviour in hunter–gatherers

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 indivi...

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Bibliographic Details
Published in:Proceedings of the Royal Society. B, Biological sciences Biological sciences, 2017-07, Vol.284 (1858), p.20170967-20170967
Main Authors: Samson, David R., Crittenden, Alyssa N., Mabulla, Ibrahim A., Mabulla, Audax Z. P., Nunn, Charles L.
Format: Article
Language:English
Subjects:
Actigraphy
Activity patterns
Behaviour
Chronotype
Circadian Rhythm
Conspecifics
Evolutionary Mismatch
Humans
Hunter–gatherer
Light
Natural selection
Night
Nocturnal
Predation
Sentinel
Sleep
Sleep and wakefulness
Tanzania
Variation
Vigilance
Wakefulness
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Summary: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.
ISSN:0962-8452
1471-2954
DOI:10.1098/rspb.2017.0967