Time-of-day variations in oculomotor function: I. Tonic accommodation and tonic vergence.

Time-of-day variations were studied both in tonic accommodation and in tonic vergence. Thirty-six male volunteers were measured once in the morning and at one other time on the same day. The mean baseline value of tonic accommodation was 2.02 D. Time-of-day variations were found, with a late evening measure being +0.6 D greater than the morning value. No independent time-of-day fluctuations were found in tonic vergence. Post-hoc analysis, however, showed that tonic vergence tended to shift in the same direction as an individual's tonic accommodation during the afternoon. During the evening, tonic vergences tended to shift oppositely to tonic accommodation.