Morning sunlight can phase advance the circadian rhythm of young adults

Abstract

Bright light is a know zeitgeber for the human circadian system, however, the phase shifting effects of natural sunlight are less well demonstrated. Eight male participants were exposed to 2-h periods of bright sunlight (in the morning or in the evening) over a week, following a week of restricted sunlight exposure in a repeated measures crossover design. A novel 'usual routine' methodology was used to control possible daily zeitgebers. Bright sunlight in the morning induced an advance of the acrophase of the body temperature rhythm. This study supports the salience of sunlight as an everyday zeitgeber under naturalistic conditions.