Days off and bright light: Effects on adaptation to night work

The present study examines the effects of rest days frequency and duration on bright light circadian adjustment to permanent night work. A sample of 12 subjects, randomly divided into 2 groups (A, B), worked between 2300 and 0700 during 12 days interrupted by 2 days off every 6 days on (system A) or by 1 day off every 3 days on (system B). All subjects received 2500-3000 Ix between 0200 and 0500 during only the 3 first days of night work. During the recovery after the 12 nightshifts, :; cycles of bright light were administered between 1000 and 1300 for all subjects. The results indicated that there is a complete adjustment to night work and no days-off effect for system A subjects who were equally divided into evening and indifferent type. For system B, only two subjects adjusted completely to night work whereas three subjects presented a partial adjustment and one showed no adjustment. The two subjects 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 related to the time of bright-light exposure: the exposure period between 0200 and 0500 a.m. may be more favourable for the subjects with late onset melatonin production. During the days-off period, the systems A and B subjects slept during the nocturnal period but no turn back was observed for those with a complete adjustment and a slight turn back occurred for those with partial adjustment. These results suggest first that there is no turnback when the adjustment is complete. Secondly, one day off is insufficient to induce a complete turn back even when the adjustment to night work is partial.