Phase-shifting human circadian rhythms with exercise during the night shift.

Appropriately timed exercise can phase shift the circadian rhythms of rodents. The purpose of this study was to determine whether exercise during the night shift could phase delay the temperature rhythm of humans to align with a daytime sleep schedule. Exercise subjects (N = 8) rode a stationary cycle ergometer for 15 min every h during the first 3 of 8 consecutive night shifts, whereas control subjects (N = 8) remained sedentary. All subjects wore dark welder's goggles when outside after the night shift until bedtime, and then slept in dark bedrooms. Sleep was delayed 9 h from baseline. Rectal temperature was continuously measured. There were fewer evening-types and more morning-types in the exercise group than in the control group, which should have made phase delay shifts more difficult for the exercise group. Nevertheless, a majority of the exercise subjects (63%) had large temperature rhythm phase delay shifts ( > 6 h in the last 4 days relative to baseline), whereas only 38% of the control subjects had large shifts. An ANCOVA showed that, when morningness-eveningness was accounted for (as the covariate), the exercise group had a significantly larger temperature rhythm phase shift than the control group. As expected, there was a correlation between the temperature rhythm phase shift and morningness-eveningness in the control group, with greater eveningness resulting in larger phase shifts. However, there was no such relationship in the exercise group; exercise facilitated temperature rhythm phase shifts regardless of circadian type. These results suggest that exercise might be used to promote circadian adaptation to night shift work.