Night shift performance is improved by a compromise circadian phase position: study 3. Circadian phase after 7 night shifts with an intervening weekend off.

STUDY OBJECTIVE: To produce a compromise circadian phase position for permanent night shift work in which the sleepiest circadian time is delayed out of the night work period and into the first half of the day sleep period. This is predicted to improve night shift alertness and performance while permitting adequate late night sleep on days off. DESIGN: Between-subjects. SETTING: Home and laboratory. PARTICIPANTS: 24 healthy subjects. INTERVENTIONS: Subjects underwent 3 simulated night shifts, 2 days off, and 4 more night shifts. Experimental subjects received five, 15 minute bright light pulses from light boxes during night shifts, wore dark sunglasses when outside, slept in dark bedrooms at scheduled times after night shifts and on days off, and received outdoor afternoon light exposure (the "light brake"). Control subjects remained in normal room light during night shifts, wore lighter sunglasses, and had unrestricted sleep and outdoor light exposure. MEASUREMENTS AND RESULTS: The final dim light melatonin onset (DLMO) of the experimental group was approximately approximately 04:30, close to our target compromise phase position, and significantly later than the control group at approximately 00:30. Experimental subjects performed better than controls, and slept for nearly all of the allotted time in bed. By the last night shift, they performed almost as well during the night as during daytime baseline. Controls demonstrated pronounced performance impairments late in the night shifts, and exhibited large individual differences in sleep duration. CONCLUSIONS: Relatively inexpensive and feasible interventions can produce adaptation to night shift work while still allowing adequate nighttime sleep on days off.