ALERTNESS AND PERFORMANCE DURING NIGHT WORK CAN BE SERIOUSLY IMPAIRED.1,2 THIS OCCURS BECAUSE THE MASTER CIRCADIAN CLOCK OF MOST night workers, which controls the body�s circadian rhythms

1481 ALERTNESS AND PERFORMANCE DURING NIGHT WORK CAN BE SERIOUSLY IMPAIRED.1,2 THIS OCCURS BECAUSE THE MASTER CIRCADIAN CLOCK OF MOST night workers, which controls the body�s circadian rhythms (e.g., alertness, temperature, melatonin), does not shift to realign with a night work, day sleep schedule.3 A sharp increase in sleepiness and decrease in performance occurs around the minimum of the circadian rhythm of body temperature (Tmin), which is usually during the night shift.4,5 Even with adequate daytime sleep, night shift decrements remain if the circadian clock is not shifted (e.g., Sharkey et al.6). Consequently, night work is associated with safety risks for both the individual worker as well as society.1,2 Alertness and performance during night shifts can be improved by stimulants such as caffeine7 and modafinil.8,9 Bright light exposure during night shifts can also improve alertness via its direct alerting effect.10 Short naps may also be useful for reducing the decrements in night shift alertness.7 However, none of these interventions can overcome the nadir in the circadian rhythm of alertness.7,11 These countermeasures do not address the underlying cause of the problem, which is misalignment between circadian rhythms and the sleep and work schedule. Laboratory and field studies of night work have shown that scheduled exposure to bright light and darkness (sleep) can be used to shift the circadian clock to completely align with a night work, day sleep schedule.12-16 Complete re-entrainment greatly improves alertness and performance during night shifts.12,17 Despite the appeal of complete re-entrainment from an alertness and safety perspective, few night workers are likely to adopt it because the slowness with which the circadian clock adjusts precludes shifting back to a diurnal schedule on days off. We have thus assessed the feasibility of a compromise sleep schedule combined with interventions to delay the circadian clock to only partially entrain to the night work, day sleep schedule. The goal of partial re-entrainment is to delay the sleepiest circadian time out of the night work period, into the first portion of the daytime sleep episodes after work, as well as to maintain it near the end of late nighttime sleep episodes on days off. When a compromise phase position is maintained throughout alternations between night shifts and days off, it is conducive of afternoon and evening alertness on days off, as well as alertness during night shifts. In a series of 4 studies with alternating blocks of night shifts and days off,18-21 we defined the target compromise circadian phase position as a dim light melatonin onset (DLMO) of 3:00. At this phase, the Tmin, an estimate of the sleepiest circadian time, which occurs about 7 h after the DLMO,22�24 will fall at ~10:00. The sleepiest circadian time would thus be early in the daytime sleep episodes after night work (daytime sleep started at 8:30) and late in the sleep episodes on days off (sleep started at 3:00). The last 2 studies of this series showed that scheduled exposure to bright light and darkness (sleep) delayed the circaCIRCADIAN ADJUStMENt AND PERMANENt NIGHt WORK