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 the mammalian eye, which could account for fluctuating sensitivity to light over the day in certain species. However, very few studies have been devoted 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 that the visual detection threshold changes over the 24h period, with high thresholds 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 suggest that a circadian rhythm influences visual sensitivity to mesopic luminance in humans.

Human core temperature minimum can be modified by ambient thermal transients.

By using slow linear thermal transients (+/- 0.025 degree C/min) of reduced amplitude (+/- 3 degrees C around thermoneutrality), we were able to advance the minimum of human internal temperature (Ti) during nocturnal sleep. During experimental night the minimum of esophageal (Tes) and rectal (Tre) temperature were respectively advanced by 1.6 h (P < 0.01) and 2.6 h (P < 0.001) in comparison to reference night spent at thermoneutrality. It must be emphasized that the provoked advance of nocturnal Ti minimum was not accompanied by any change in sleep latency, efficiency, SWS and REM sleep percentages. The result shows that appropriate ambient temperature transient changes could be used to modify the course of human Ti one of the major biological rhythms usually considered as resistant to sleep-wake rhythm manipulation.

Effects of noise on sleep inertia as a function of circadian placement of a one-hour nap.

The purpose of the present study was to analyse the arousing effects of noise on sleep inertia as a function of circadian placement of a one-hour nap. In a first experiment, we measured the effects of sleep inertia in a neutral acoustic environment after a one-hour nap placed either at 0100 or 0400 on response time during a spatial memory test. In a second experiment were analysed the effects of an intense continuous noise on sleep inertia. The results showed that noise produced a total abolition of sleep inertia after an early nap (0000 to 0100). This may be due to the arousing effect of noise; however, results are less clear after a late nap 0300 to 0400 as noise seems to be ineffective. This result is discussed in terms of either a function of time-of-day effect or of prior sleep intensity. Moreover, our data suggest a possible interaction of noise with partial sleep deprivation leading to a slight deleterious effect those subjects who did not sleep at all.