Bright Light Resets the Human Circadian Pacemaker Independent of the Timing of the Sleep-Wake Cycle

TLDR

Human circadian rhythms were once considered insensitive to light, with 24‑hour alignment thought to arise from social contacts or sleep schedules, but recent evidence of an intensity‑dependent neuroendocrine response to bright light suggests light can act as a synchronizer. The study monitored an elderly woman’s circadian output before and after seven consecutive evenings of 4‑hour bright‑light exposure, and compared it to a control period of ordinary room light while her sleep‑wake schedule and social contacts remained unchanged. Evening bright‑light exposure produced a 6‑hour delay shift in her circadian pacemaker, as shown by body‑temperature and cortisol rhythms, demonstrating that bright light can rapidly and stably reset the human circadian system.

Abstract

Human circadian rhythms were once thought to be insensitive to light, with synchronization to the 24-hour day accomplished either through social contacts or the sleep-wake schedule. Yet the demonstration of an intensity-dependent neuroendocrine response to bright light has led to renewed consideration of light as a possible synchronizer of the human circadian pacemaker. In a laboratory study, the output of the circadian pacemaker of an elderly woman was monitored before and after exposure to 4 hours of bright light for seven consecutive evenings, and before and after a control study in ordinary room light while her sleep-wake schedule and social contacts remained unchanged. The exposure to bright light in the evening induced a 6-hour delay shift of her circadian pacemaker, as indicated by recordings of body temperature and cortisol secretion. The unexpected magnitude, rapidity, and stability of the shift challenge existing concepts regarding circadian phase-resetting capacity in man and suggest that exposure to bright light can indeed reset the human circadian pacemaker, which controls daily variations in physiologic, behavioral, and cognitive function.