Data from: 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 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 minutes in total over 20 days of observation, with a median of eight individuals awake throughout the nighttime 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.睡眠对生存至关重要，但同时也是一段极易受到捕食者、敌对同类以及环境危险威胁的脆弱时期。为降低睡眠期间的风险，哨兵假说（sentinel hypothesis）提出：群居动物会在睡眠时段分担警戒任务，即部分个体睡眠时，另有个体保持清醒。为探究人类睡眠时是否存在类似哨兵的行为，我们对坦桑尼亚哈德扎狩猎采集者（Hadza hunter-gatherers）的夜间活动模式展开了研究。通过腕动计（actigraphy）监测，我们发现，在20天的观测周期内，所有受试者同时处于睡眠状态的总时长仅为18分钟；夜间始终保持清醒的个体中位数为8人。因此，从第一人入睡到最后一人醒来的时段内，99.8%的采样时段中均有至少1名个体保持清醒（或处于浅睡眠阶段）。我们证实，这种活动水平的异步性源于睡眠时型（chronotype）的差异，且睡眠时型会随年龄发生变化。由此可见，群体睡眠时，清醒时段的异步性可为警戒行为提供条件。我们据此提出：在人类演化历程中，由不同年龄个体组成的睡眠群体，曾承担了一定的警戒功能。因此，现代人群中存在的睡眠时型差异与人类睡眠结构（sleep architecture，包括夜间觉醒），或许是过去自然选择为降低睡眠风险而留下的演化遗留特征。