Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans.

CONTEXT Millions of individuals habitually expose themselves to room light in the hours before bedtime, yet the effects of this behavior on melatonin signaling are not well recognized. OBJECTIVE We tested the hypothesis that exposure to room light in the late evening suppresses the onset of melatonin synthesis and shortens the duration of melatonin production. DESIGN In a retrospective analysis, we compared daily melatonin profiles in individuals living in room light (<200 lux) vs. dim light (<3 lux). PATIENTS Healthy volunteers (n = 116, 18-30 yr) were recruited from the general population to participate in one of two studies. SETTING Participants lived in a General Clinical Research Center for at least five consecutive days. INTERVENTION Individuals were exposed to room light or dim light in the 8 h preceding bedtime. OUTCOME MEASURES Melatonin duration, onset and offset, suppression, and phase angle of entrainment were determined. RESULTS Compared with dim light, exposure to room light before bedtime suppressed melatonin, resulting in a later melatonin onset in 99.0% of individuals and shortening melatonin duration by about 90 min. Also, exposure to room light during the usual hours of sleep suppressed melatonin by greater than 50% in most (85%) trials. CONCLUSIONS These findings indicate that room light exerts a profound suppressive effect on melatonin levels and shortens the body's internal representation of night duration. Hence, chronically exposing oneself to electrical lighting in the late evening disrupts melatonin signaling and could therefore potentially impact sleep, thermoregulation, blood pressure, and glucose homeostasis.