Ancestral Sleep Patterns and the Modern Insomnia Epidemic

The assumption that pre-industrial humans slept dramatically more than modern populations has been challenged by recent cross-cultural research. Studies of the Hadza, San, and Tsimané — contemporary hunter-gatherer and small-scale agricultural societies — reveal sleep durations of approximately 5.7–7.1 hours per night, comparable to or shorter than industrialized populations.

What Differs Is Quality, Not Quantity

While total sleep time may be similar, the sleep architecture and environmental conditions differ substantially:

• Temperature tracking: Sleep onset in these populations correlates tightly with the post-sunset drop in ambient temperature, not with sunset itself. This implicates thermoregulation as a primary sleep onset cue.
• Light exposure patterns: Daytime light exposure averages 10–20x higher than typical indoor workers receive, with a sharp contrast to near-total darkness after sunset.
• Consolidated sleep: Insomnia complaints are remarkably rare in these populations. The Hadza language reportedly lacks a word for insomnia.

The Light–Temperature Mismatch

Modern environments invert the ancestral pattern in two critical ways:

1. Insufficient daytime light exposure (most indoor environments provide 100–500 lux vs. 10,000+ lux outdoors)
2. Excessive evening light exposure (screens, artificial lighting) coupled with stable indoor temperatures

This combination disrupts both the photic and non-photic entrainment pathways for the circadian system.

Melatonin and Temperature

In natural conditions, melatonin onset occurs approximately 2 hours before sleep onset, coinciding with the evening temperature decline. In modern environments, evening light suppresses melatonin while stable indoor temperatures eliminate the thermoregulatory sleep cue.

Practical Synthesis

The evolutionary lens suggests that sleep interventions should focus on:

• Maximizing daytime bright light exposure (ideally outdoor light in the morning)
• Minimizing evening artificial light, particularly blue-enriched sources
• Allowing bedroom temperature to drop (the evidence supports 65–68°F / 18–20°C)
• Establishing consistent sleep–wake timing to support circadian entrainment

These interventions address root causes rather than symptoms, and align with what the available cross-cultural evidence suggests about human sleep biology.