Variation of visual detection over the 24-hour period in humans

A circadian rhythm for visual sensitivity has been intensively assessed in animals. This rhythm may be due to the existence of a circadian clock in th e mammalian eye, which could account, for fluctuating sensitivity to light over the day in certain species. However, very few studies have been devote d to the human visual system. The present experiment was designed to assess a possible rhythm of visual sensitivity using a psychophysical method over the whole 24h period. Twelve subjects underwent visual detection threshold measures in a protocol that allowed one point every 2h. The results show t hat the visual detection threshold changes over the 24h period, with high t hresholds in the morning, a progressive decrease over the day and the early night, and an increase during the last part of the night. These data sugge st that a circadian rhythm influences visual sensitivity to mesopic luminan ce in humans.