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

Abstract

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.

Trial registration: ClinicalTrials.gov NCT00200863.

Fig. 1.

Melatonin onset occurs later in room light than in dim light. A, In study 1, participants lived in room light (<200 lux) and slept in darkness for the first two baseline days. Upon awakening on the morning of d 3, participants were exposed to 8 h of room light, followed by 8 h of dim light (<3 lux) before bedtime. After sleep, individuals underwent a constant routine procedure in dim light. B, The melatonin rhythm is shown for a representative subject over three consecutive cycles (d 2–5). In this subject, melatonin onset occurred about 2 h earlier on d 3 and 4 in dim light, compared with d 2 in room light. The timing of melatonin onset and offset are shown by the labeled arrows in gray. White and gray bars at the top of each plot indicate exposure to room light and dim light, respectively, and black bars indicate sleep in darkness.

Fig. 2.

Exposure to room light before bedtime shortens melatonin (MLT) duration. A, Histograms show the timing of melatonin onset (gray bars) and offset (white bars) in participants (n = 104) living in room light vs. dim light. White and gray bars at the top of each plot indicate exposure to room light (<200 lux) and dim light (<3 lux), respectively, and black bars indicate scheduled sleep in darkness. B, Histograms show melatonin duration in the same participants over three consecutive cycles corresponding to A. Median melatonin duration is indicated by the vertical line with label. Melatonin duration is longest when the onset and offset occur under dim light. C, Horizontal bar chart showing the change in timing of melatonin onset in individual participants from d 2 in room light to d 3 in dim light. In 99% of individuals, melatonin onset occurred earlier in dim light relative to room light. Data are ranked in ascending order of magnitude. D–F, Similar plots are shown for changes in the timing of melatonin offset from the morning of d 3 in room light to d 4 in dim light (D), melatonin onset from d 3 in dim light to d 4 in dim light (E), and melatonin offset from the morning of d 4–5 in dim light (F).

Fig. 3.

Exposure to room light before bedtime decreases melatonin (MLT) levels. A, Histogram showing the percent decrease in melatonin from melatonin onset to bedtime when participants were exposed to room light (<200 lux) vs. dim light (<3 lux) until scheduled sleep. The AUC of the melatonin profile was determined in dim light and compared with the AUC on the preceding day at the same relative clock times. Median percent decrease in melatonin is indicated by the vertical line with label. B, The melatonin rhythm is shown for three representative volunteers exposed to room light (○) before and after scheduled sleep in darkness (enclosed by the vertical dashed lines) vs. exposure to dim light (●) at the same relative clock times on the following day.

Fig. 4.

Exposure to room light elicits strong suppression of melatonin during the usual hours of sleep. A, Participants lived in ambient room light (<200 lux) and slept in darkness for 3 baseline days, after which they underwent a constant routine procedure in room light (<200 lux). Data are shown from either d 6–9 or 10–13 of a 14-d research protocol (See Subjects and Methods). B, The melatonin rhythm is shown for five individuals exposed to room light (○) during the constant routine vs. darkness during sleep (●) on the preceding day. Percent melatonin suppression by room light is indicated at the top of each plot for the 8 h corresponding to habitual sleep (enclosed by the vertical dashed lines).