IMPROVING ADAPTATION TO SIMULATED NIGHT-SHIFT - TIMED EXPOSURE TO BRIGHT LIGHT VERSUS DAYTIME MELATONIN ADMINISTRATION

Chronic circadian disturbance is thought to cause many of the health a nd social problems reported by shift workers. In recent years, appropr iately timed exposure to bright light and exogenous melatonin have bee n used to accelerate adaptation to phase shifts of the circadian syste m. In this study we compared adaptation to night shift in three groups of subjects. The first treatment group received timed exposure to bri ght light (4-7,000 lux between 2400 and 0400 hours on each of three ni ght shifts). The second treatment group received exogenous melatonin b y capsule (2 mg at 0800 hours then 1 mg at 1100 and 1400 hours). The p lacebo control groups received either dim red light at less than 50 lu x or placebo (sucrose) in identical capsules at the same time. Results indicated that all groups shifted significantly from baseline. Using the dim-light melatonin onset as a circadian marker, the bright-light group shifted the furthest, whereas there was no significant differenc e between the melatonin and placebo groups. Sleep quality as determine d by wrist actigraphy was most improved in the light-treatment group, although the melatonin group also showed significant improvements. Cog nitive psychomotor performance was most improved in the light-treatmen t group and the melatonin group again showed little difference from th e control group. Although melatonin was unable to increase the amount of the phase shift following transition to night shift, it is Likely t hat the intermediate levels of improvement in sleep reflect the hypoth ermic effects of melatonin. By lowering core temperature across the sl eep period, sleep may be enhanced. This improvement in sleep quality d id not produce concomitant improvements in shift performance for the m elatonin group. This suggests that the enhanced performance in the lig ht-treatment group may reflect more direct ''energizing'' effects. On the basis of these results, bright light is clearly superior in its ab ility to phase shift the circadian system and thereby improve sleep an d performance. However, melatonin may permit shift workers to override the circadian system for short periods and avoid the potential toxici ty due to overzealous manipulations of the circadian pacemaker. In rap idly rotating shift schedules, melatonin may be preferable because it would not require workers to reverse the large phase shift induced by light.