Does the Moon Influence Sleep in Small-scale Societies?

Objectives: The lunar cycle is expected to influence sleep-wake patterns in human populations that have greater exposure to the environment, as might be found in forager populations that experience few environmental buffers. We investigated this "moonlight" hypothesis in two African populations: one composed of hunter-gatherers (with minimal environmental buffering) and the other rural agriculturalists (with low-to-moderate environmental buffering).

Setting: Research was conducted on Hadza hunter-gatherers from the Sengele community near Lake Eyasi in northern Tanzania and in Mandena, Madagascar, in a rural community of approximately 4000 farmers.

Participants: Thirty-one adult Hadza and 21 Malagasy adults were recruited.

Measurements: We used the CamNtech Motionwatch 8 actigraph and generated data on an epoch-by-epoch, 1-minute basis.

Results: In general support of the moonlight hypothesis, we uncovered an association between sleep-wake patterns and lunar cycle (ie., moonlight) for Hadza hunter-gatherers. However, the direction of the effect was opposite to what we predicted: as the potential for exposure to moonlight increased, activity generally shifted to a pattern of less nighttime activity and greater daytime activity. No significant effects were found in the Malagasy agriculturalists.

Conclusions: The proposal that human behaviors are linked with moon phase is a popular belief that persists despite the absence of consistent evidence. We provide the first direct evidence that lunar cycle is linked to sleep-wake pattern in a hunter-gatherer society, suggesting that moonlight does not inhibit sleep-wake patterns in the ways that electric lighting does.