Shaping the light/dark pattern for circadian adaptation to night shift work.

This is the second in a series of simulated night shift studies designed to achieve and subsequently maintain a compromise circadian phase position between complete entrainment to the daytime sleep period and no phase shift at all. We predict that this compromise will yield improved night shift alertness and daytime sleep, while still permitting adequate late night sleep and daytime wakefulness on days off. Our goal is to delay the dim light melatonin onset (DLMO) from its baseline phase of approximately 21:00 to our target of approximately 3:00. Healthy young subjects (n=31) underwent three night shifts followed by two days off. Two experimental groups received intermittent bright light pulses during night shifts (total durations of 75 and 120 min per night shift), wore dark sunglasses when outside, slept in dark bedrooms at scheduled times after night shifts and on days off, and received outdoor light exposure upon awakening from sleep. A control group remained in dim room light during night shifts, wore lighter sunglasses, and had unrestricted sleep and outdoor light exposure. After the days off, the DLMO of the experimental groups was approximately 00:00-1:00, not quite at the target of 3:00, but in a good position to reach the target after subsequent night shifts with bright light. The DLMO of the control group changed little from baseline. Experimental subjects performed better than control subjects during night shifts on a reaction time task. Subsequent studies will reveal whether the target phase is achieved and maintained through more alternations of night shifts and days off.