Diurnal rhythmicity for visual sensitivity in humans?

Visual sensitivity rhythm has been assessed by several authors in animals. It shows an intrinsic circadian rhythm of some retinal mechanisms that could account for fluctuating sensitivity to light during the day in these species. However, very little is known concerning a possible circadian rhythm of visual sensitivity in humans. The present experiment was designed to assess a diurnal rhythm of visual sensitivity using psychophysical methods. Two different detection threshold measures (adaptive and constant methods) were applied on 7 highly entrained subjects. The results show a strongly increased visual detection threshold in the morning (8:00 h) in 4 of 7 subjects, followed by an important improvement at 10:00 h, after which time it remains almost constant whatever the method used. But 3 subjects had constant thresholds throughout the day. A "first session" effect as well as a "chronotype" effect were ruled out. The results are discussed in terms of a possible effect of sleep inertia, suggesting a long-lasting effect probably dependent on the type of task.