Intra-ocular pressure (IOP) was measured immediately after normal subjects were woken from at least 5 hours sleep. Measurements were made at approximately 15 s intervals, for about 20 minutes. The IOP of all 14 subjects was elevated after sleep and returned to baseline levels with a time course which was approximately exponential; the longest time constant of return of IOP to baseline was 1056.9 s, and the shortest 133.5 s. Mean time constant of recovery was 404.8 s. The decrease in IOP may be related to melatonin levels which increase during sleep and decrease in the light, or be related to accommodation and eye movements which may act to 'pump' aqueous from the eye.