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

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.

Sleep in a comparative context: Investigating how human sleep differs from sleep in other primates

“…Future research could investigate the specific stages of NREM that shifted over evolutionary time. Based on recent evidence that variability in human chronotype may have increased group level vigilance during nighttime periods by way of sentinel‐like behavior (Samson, Crittenden, Mabulla, Mabulla, & Nunn, ), we predict that the lightest stages of NREM sleep (stage 1 and 2, where arousal threshold is low) proportionally decreased relative to deep NREM slow wave sleep (stage 3, where arousal threshold is high).…”

Sleep variability and nighttime activity among Tsimane forager‐horticulturalists

“…Few sleep studies have been conducted in non‐electrified communities, and even fewer among small‐scale subsistence populations. The significance of sleep in evolutionary and ecological contexts has generated increased attention recently, however, and thus more observational sleep studies in field populations have been conducted in the past few years than ever before (Beale et al, ; de la Iglesia et al, ; Knutson, ; Moreno et al, ; Samson, Crittenden, Mabulla, & Mabulla, , Samson, Crittenden, Mabulla, Mabulla, & Nunn, , 2017d, Samson et al, ; Yetish et al, ). While these studies have contributed novel insights, they focused on addressing variation in sleep among individuals or groups.…”

The cost of deep sleep: Environmental influences on sleep regulation are greater for diurnal lemurs

“…The next significant improvement in sleeping site could have been the tree‐to‐ground transition, which likely occurred with early Homo given the dramatic morphological changes that took place during the Australopithecus‐Homo transition (Coolidge & Wynn, ). This evolutionary event could have then established the prerequisite adaptations to alter early hominin sleep architecture, where hominins would have benefited from more stable and less thermodynamically stressful sleeping sites (Samson & Hunt, ), and could have combined shelter and bedding technology (Samson, Crittenden, Mabulla, Mabulla, & Nunn, ) and group level social cohesion, promoting sentinel‐like behavior (Samson, Crittenden, Mabulla, & Mabulla, ) to improve sleep intensity as a result of greater comfort and security at sleeping sites.…”