DAYS OFF AND BRIGHT LIGHT - EFFECTS ON ADAPTATION TO NIGHT WORK

The present study examines the effects of rest days frequency and dura tion on bright light circadian adjustment to permanent night work. A s ample of 12 subjects, randomly divided into 2 groups (A, B), worked be tween 2300 and 0700 during 12 days interrupted by 2 days off every 6 d ays on (system A) or by 1 day off every 3 days on (system B). All subj ects received 2500-3000 Ix between 0200 and 0500 during only the 3 fir st days of night work. During the recovery after the 12 nightshifts, : ; cycles of bright light were administered between 1000 and 1300 for a ll subjects. The results indicated that there is a complete adjustment to night work and no days-off effect for system A subjects who were e qually divided into evening and indifferent type. For system B, only t wo subjects adjusted completely to night work whereas three subjects p resented a partial adjustment and one showed no adjustment. The two su bjects who adjusted completely were evening type and presented a phase delay of their baseline of aMT6s rhythm compared to the subjects who partially adjusted. These differences in adjustment are probably relat ed to the time of bright-light exposure: the exposure period between 0 200 and 0500 a.m. may be more favourable for the subjects with late on set melatonin production. During the days-off period, the systems A an d B subjects slept during the nocturnal period but no turn back was ob served for those with a complete adjustment and a slight turn back occ urred for those with partial adjustment. These results suggest first t hat there is no turnback when the adjustment is complete. Secondly, on e day off is insufficient to induce a complete turn back even when the adjustment to night work is partial.