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ABSTRACT
This study tested the hypotheses that 2-h exposure to morning bright light after waking may avoid a delay in nadir time for individuals who experience delayed nadir time after waking and may improve the morning rise in core temperature (Tc) due to residual effects of morning bright light on body temperature regulation. Eight young men were exposed to 110, 1000, and 4000 lx light for 2 h after waking at 7:00 AM. After 2-h exposure to a given illuminance, illuminance was controlled at 1000 lx until 12:00 PM. Rectal temperature (Tre) was measured as a proxy for Tc continuously from 12:00 AM on the prior night to 12:00 PM on the day of the experiment. To estimate body heat balance, skin temperatures and values for regional dry heat loss at seven sites were continuously measured from 8:00 AM to 12:00 PM, together with measurement of oxygen consumption and finger skin blood flow rate (SkBF) from 8:00 AM to 12:00 PM at 1-h intervals. Results indicated delays of a few minutes in nadir time with 1000 and 4000 lx, as opposed to a 120-min delay with 110 lx. In addition, Tre increased after 2-h exposure to 4000 lx light, probably due to the significantly (P < 0.01) lower mean skin temperature, SkBF, and dry heat loss compared to the other lighting conditions. Light exposure over 1000 lx may thus avoid substantial delays in nadir time, and light exposure over 4000 lx could restore Tc rhythm in a prompt manner due to residual effects of morning bright light on body temperature regulation.