S. Bougrine et al., DAYS OFF AND BRIGHT LIGHT - EFFECTS ON ADAPTATION TO NIGHT WORK, International journal of industrial ergonomics, 21(3-4), 1998, pp. 187-198
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.