The effect of light on core body temperature is mediated by melatonin in women.

Acute exposure to bright light at night reduces the nocturnal decline of core body temperature (cBT) and inhibits melatonin secretion in men. Since inhibition of melatonin secretion by beta-adrenergic blockade reduces the nocturnal decline of cBT by 40\% in women, experiments were performed to investigate whether the thermoregulatory effect of light is mediated by modifications of melatonin secretion in cycling women. Results show that the elevation of cBT induced by nocturnal exposure to bright light (3000 lux) can be reversed completely by circumventing the decline of serum melatonin levels with concurrent oral administration of melatonin. Our finding establishes melatonin as the mediator of the effect of light on cBT in women and provides a rationale for the use of orally administered melatonin as an aid in the reentrainment of the cBT rhythm in desynchronized conditions.