Preliminary studies on the immediate phase-shifting effects of light and exercise on the human circadian clock

The aim of the present research was to determine the magnitude and direction of immediate phase shifts of human rhythms following a single exposure to a 3-hr pulse of bright light or physical activity. The pulse of light or activity was presented under "constant-routine" conditions, and measurements of the resultant phase shifts were performed under the same constant-routine conditions on the first day following pulse presentation. Four overt rhythms that are strongly dependent on circadian timing--namely, the rhythms of plasma cortisol, plasma thyroid-stimulating hormone (TSH), plasma melatonin, and body temperature--were monitored. The analysis of the TSH profiles indicated that exposure to light at about the time of the minimum of body temperature resulted in phase advances averaging less than 1 hr in magnitude. Exposure to light approximately 3 hr before the time of the minimum of body temperature resulted in phase delays of 1-2 hr. Preliminary analyses of the melatonin profiles have confirmed these observations. Our findings regarding the effects of exercise are still inconclusive.