Effects of timed bright-light exposure on shift-work adaptation in middle-aged subjects.

Shift workers suffer from a constellation of symptoms that can severely compromise their ability to perform optimally on-shift. The largest single factor contributing to shift-worker problems is sleep disturbance, and there is little question that the primary cause of such sleep disturbance is circadian disruption. Recently, a number of studies have demonstrated that timed exposure to bright light can help facilitate adaptation to simulated shift-work schedules, at least in younger subjects. The aim of the current study was to assess the effects of bright-light interventions in middle-aged individuals undergoing a simulated shift-work regimen. Results indicate that although light was effective in resetting the circadian clocks of these subjects by more than 6 hours, there was little effect on measures of on-duty alertness and performance or on off-duty sleep. These findings suggest that middle-aged subjects may be less phase-tolerant than young subjects, and they raise questions concerning the utility of bright-light interventions in some shift-work populations.