Daytime naps in darkness phase shift the human circadian rhythms of melatonin and thyrotropin secretion

To systematically determine the effects of daytime exposure to sleep in dar kness on human circadian phase, four groups of subjects participated in 4-d ay studies involving either no nap (control), a morning nap (0900-1500), an afternoon nap (1400-2000), or an evening nap (1900-0100) in darkness. Exce pt during the scheduled sleep/dark periods, subjects remained awake under c onstant conditions, i.e., constant dim light exposure (36 lx), recumbence, and caloric intake. Blood samples were collected at 20-min intervals for 64 h to determine the onsets of nocturnal melatonin and thyrotropin secretion as markers of circadian phase before and after Stimulus exposure. Sleep wa s polygraphically recorded. Exposure to sleep and darkness in the morning r esulted in phase delays, whereas exposure in the evening resulted in phase advances relative to controls. Afternoon naps did not change circadian phas e. These findings indicate that human circadian: phase is dependent on the timing of darkness and/or sleep exposure and that strategies to treat circa dian misalignment should consider not only the timing and intensity of ligh t, but also the timing of darkness and/or sleep.