Resetting of circadian melatonin and cortisol rhythms in humans by ordinary room light

A weak photic stimulus significantly phase-advanced the plasma melatonin rhythm in eight healthy young men.

• Exposure to ordinary room light (approximately 180 lux) was applied through three cycles of 5-hour sessions.
• Control subjects exposed to darkness exhibited phase delays in their plasma melatonin rhythms.
• The phase advancement of plasma melatonin was statistically significant (p < or = 0.003).
• Following light exposure, the rhythms of plasma melatonin and plasma cortisol remained aligned with the core body temperature cycle.
• These findings suggest that ordinary indoor light can influence the master circadian pacemaker.

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