Episode 24 explores the history of first and second sleep, then compares it with measured sleep in three contemporary non-industrial communities and an eight-person Colorado camping experiment. Historical patterns and plausible environmental explanations are discussed as interpretations, not prescriptions for everyone.

Research notes: actigraphy estimates sleep rather than directly measuring brain stages. The Yetish study found 5.7–7.1 hours of estimated sleep within 6.9–8.5-hour sleep periods; it does not establish an ideal sleep duration for an individual. Temperature associations do not prove that a thermostat causes insomnia. The Tsimane are hunter-horticulturalists. The camping study measured circadian timing, not a universal cure. Persistent insomnia or daytime impairment deserves qualified medical care.

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Sources for this reviewed edition:

Roger Ekirch — Segmented Sleep: historical references

https://sites.google.com/vt.edu/roger-ekirch/sleep-research/segmented-sleep

Yetish et al. (2015) — Natural sleep and its seasonal variations in three pre-industrial societies

https://pmc.ncbi.nlm.nih.gov/articles/PMC4720388/

Wright et al. (2013) — Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle

https://pmc.ncbi.nlm.nih.gov/articles/PMC4020279/