Sleep timing and duration in indigenous villages with and without electric lighting on Tanna Island, Vanuatu

Andrea N. Smit,Tanya Broesch,Ralph E. Mistlberger,Jerome M. Siegel

doi:10.1038/s41598-019-53635-y

Abstract

It has been hypothesized that sleep in the industrialized world is in chronic deficit, due in part to evening light exposure, which delays sleep onset and truncates sleep depending on morning work or school schedules. If so, societies without electricity may sleep longer. However, recent studies of hunter-gatherers and pastoralists living traditional lifestyles without electricity report short sleep compared to industrialized population norms. To further explore the impact of lifestyles and electrification on sleep, we measured sleep by actigraphy in indigenous Melanesians on Tanna Island, Vanuatu, who live traditional subsistence horticultural lifestyles, in villages either with or without access to electricity. Sleep duration was long and efficiency low in both groups, compared to averages from actigraphy studies of industrialized populations. In villages with electricity, light exposure after sunset was increased, sleep onset was delayed, and nocturnal sleep duration was reduced. These effects were driven primarily by breastfeeding mothers living with electric lighting. Relatively long sleep on Tanna may reflect advantages of an environment in which food access is reliable, climate benign, and predators and significant social conflict absent. Despite exposure to outdoor light throughout the day, an effect of artificial evening light was nonetheless detectable on sleep timing and duration.

Highlights

Sleep timing and duration in humans are determined in part by a master circadian clock entrained to local time by retinal inputs encoding environmental light-dark (LD) cycles[1]
We found that habitual sleep duration among the Ni-Vanuatu of Tanna Island is long compared to several small-scale hunter-gatherer, agrarian and pastoralist societies in Africa and Bolivia[27,28,29,30], and compared to most samples from industrialized western populations studied by actimetry using Actiwatches (Fig. 10)
We found that nocturnal sleep onset was delayed by 23 minutes and duration was shorter by 28 minutes in participants living in coastal villages with on-demand access to electric light at night

Summary

Introduction

Sleep timing and duration in humans are determined in part by a master circadian clock entrained to local time by retinal inputs encoding environmental light-dark (LD) cycles[1]. Epidemiological studies have uncovered associations between short sleep (≤~6 h) and population health, while experimental studies support a causal role for sleep restriction in metabolic and other health disorders currently described as epidemic[9,10,11,12,13,14] This leads to conjecture that a significant portion of the population in industrialized societies may sleep less than is physiologically optimal (estimated to be 7 h for adults14) and that this may contribute to negative trends in population health[15,16,17]. Trends over the past several decades within already industrialized societies are equivocal, with some studies showing increases[21,22,23], others showing decreases[24], and some showing no systematic change in sleep length[25,26] Another way to estimate the impacts of industrialization on sleep is to study sleep in indigenous communities living traditional lifestyles without electric lighting. The latitude of Tanna Island, and the annual variation in photoperiod, is very close to the latitude of several hunter-gatherer societies previously shown to exhibit short sleep[27,30], permitting cross-cultural comparisons with a natural control for daylength

Methods

Results

Conclusion