Controlled exposure to light and darkness realigns the salivary cortisol rhythm in night shift workers

The efficacy of a light/darkness intervention designed to promote circadian adaptation to night shift work was tested in this combined field and laboratory study. Six full-time night shift workers (mean age ± SD:37.1 ± 8.1 yrs) were provided an intervention consisting of an intermittent exposure to full-spectrum bright white light (∼2000 lux) in the first 6 h of their 8 h shift, shielding from morning light by tinted lenses (neutral gray density, 15% visual light transmission), and regular sleep/darkness episodes in darkened quarters beginning 2 h after the end of each shift. Five control group workers (41.1 ± 9.9 yrs) were observed in the presence of a regular sleep/darkness schedule only. Constant routines (CR) performed before and after a sequence of ∼12 night shifts over 3 weeks revealed that treatment group workers displayed significant shifts in the time of peak cortisol expression and realignment of the rhythm with the night-oriented schedule. Smaller phase shifts, suggesting an incomplete adaptation to the shift work schedule, were observed in the control group. Our observations support the careful control of the pattern of light and darkness exposure for the adaptation of physiological rhythms to night shift work.