A quiet, unhurried meditation on the circadian rhythm — the ancient timekeeper ticking away inside every human body. From the tiny cluster of neurons that watches the light, through the shifting sleep patterns of infants, teenagers, and the elderly, to the long story of how artificial light reshaped our nights, this episode drifts gently through the biology of time itself, with a brief, drowsy visit to the animals who keep their own peculiar hours.

This episode includes AI-generated content.