Your chronotype is ~50% determined by the PER3 gene variant. About 15% are strong morning types, 20% strong evening types, 65% intermediate. Late chronotype is linked to higher insomnia and depression risk when forced to wake early. Source: Roenneberg et al. (2004); biorxiv (2025).

UK Biobank study of 88,000 adults found sleeping between 10–11 PM had lowest cardiovascular disease risk. Before 10 PM raised risk 24%; after midnight raised risk 25%. Timing is independent of sleep duration. Source: Nikbakhtian et al. (2021) European Heart Journal Digital Health.

The “forbidden zone” for sleep occurs 1–3 hrs before habitual bedtime when circadian alerting signal peaks. Trying to sleep in this window results in long sleep onset, frequent arousals, and poor sleep quality regardless of sleep pressure. Source: Dijk & Czeisler (1994) Journal of Neuroscience.

Bright light in the morning advances the circadian clock; light at night delays it. A 2024 NPR/Nature study of 90,000 people found bright nighttime light raised mortality risk 21–34%. Bright day + dark night is the strongest circadian signal. Source: Windred et al. (2024) The Lancet.

Stanford Medicine (2026) found permanent standard time causes the least circadian burden for most people — prioritising morning light. The study used mathematical modelling of light exposure from 7 million location data points. Source: Stanford Medicine (2026) via Nature.