Intraocular pressure was measured hourly during 24 hours in 12 young (20.6 +/- 0.3 years, mean +/- SEM) and 12 older (59.5 +/- 1.6 years) healthy white adults to determine whether intraocular pressure followed a circadian rhythm and whether its nocturnal variations were related to the stages of sleep in the subjects. An electronic tonometer (Tono-Pen), working on the applanation principle, which was shown to give accurate intraocular pressure measurements in any posture, was used to measure intraocular pressure. Nocturnal polysomnography was measured. Wakefulness, light sleep (stages 1 and 2), slow-wave sleep (stages 3 and 4), and rapid eye movement sleep were scored. Intraocular pressure followed a circadian rhythm with a nocturnal peak value (acrophase). The variations in intraocular pressure were related to the stage of sleep, being lowest during rapid eye movement sleep, and highest during slow-wave sleep.