We estimated diurnal and nocturnal levels of ocular perfusion pressure at rest in both young and older adults in a clinical sleep laboratory. Measurements of blood pressure and intraocular pressure (IOP) were obtained every 2 hours for 24 consecutive hours in 16 healthy young adults (ages 18-25 years) and 16 older adults (ages 47-74 years). In the 16-hour diurnal wake period, blood pressure and IOP were measured after a 5-minute sitting rest. In the 8-hour nocturnal period, measurements were taken with subjects in the supine position. Sitting and supine ocular perfusion pressures in the diurnal and nocturnal periods were calculated respectively based upon the blood pressure and IOP. Ocular perfusion pressure was found to be higher in the older group than in the younger group throughout the 24 hours. The peak of ocular perfusion pressure was in the nocturnal period for both groups. Within each subject group, the average nocturnal ocular perfusion pressure in the supine position was higher than the average diurnal ocular perfusion pressure in the sitting position. The diurnal-to-nocturnal increase of ocular perfusion pressure was larger in the older group than in the younger group.