Impact of weekly lighting condition on performance, sleepiness, and mood

Abstract

It was hypothesized that lack of daytime circadian stimulus (dim light) would delay circadian phase from the start to the end of the week, resulting in a greater decrement in performance and mood as well as an increase in sleepiness at the end of the week. Results show reduced sleepiness in high CS lighting conditions, but this effect was not increased over the course of the week. The impact of the dim light condition on performance is less clear showing limited evidence of the chronic impact of light deprivation on performance. Reaction time results indicate a more acute than chronic impact, while accuracy measures show a trend toward a greater decrement over the course of the week when subjects were deprived from daytime circadian light, but this difference did not reach significance. Memory measures do not show any significant changes due to lighting condition. Mood remained slightly better over the week in daylight than in dim light. Self-reported sleep duration increased over the course of the week. Overall results clearly indicate reduced sleepiness and a slight maintenance of mood, but the impact of daytime light on performance remains elusive.

This thesis investigates a potentially cumulative impact of blocking daytime circadian stimulus (CS) for 5 consecutive days (Monday-Friday) on performance, subjective sleepiness and mood and self-reported sleep. Biological rhythms in humans, specifically circadian rhythms (about a day), naturally run with a period slightly longer than 24 hours. They are entrained to (or synchronized with) the 24-hour day mainly by the light-dark cycle incident on the retina. For every day spent with insufficient CS, circadian rhythms become increasingly misaligned with external clock time. Chronic circadian misalignment or desynchronization, such as the ones experienced by shift workers or when traveling multiple time zones has been associated with poor sleep, mood disorders, and performance deficits.

Few studies have looked into the impact of circadian misalignment in healthy daytime workers. In the current study, twelve subjects spent eight hours a day for 5 consecutive days in three lighting conditions: 1) daylight whithout view; 2) high correlated color temperature (CCT) electric light, and 3) dim light. The daylight and high CCT light provided high CS levels while the dim light (<10 lux)) provided low CS levels. Performance tests monitoring reaction time, memory, and accuracy measures were administered daily in the morning, at lunchtime, and in the afternoon. Sleepiness ratings were collected before and after each testing period. Self-reports of mood were surveyed at the beginning and end of each week, and sleep logs were kept daily.