St. Hedwig Hospital and Charité–Universitätsmedizin Berlin researchers report that repeated mornings spent under dim indoor light in healthy young adults raised afternoon and evening cortisol and reshaped sleep in ways known from depressive illnesses.

Depressive disorders are often linked with hyperactivity of the hypothalamic–pituitary–adrenal axis, with cortisol levels that stay elevated into the afternoon and early evening instead of reaching their lowest levels, typical of early evening.

Sleep in depressive illness often carries its own fingerprint. Changes in REM sleep and a shift of slow wave sleep from the beginning of the night toward later phases have been described as biological markers of depression.