Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression.
Level 3 - non-randomized controlled study
Controlled non-randomized human experimental laboratory study
PubMed 10922269 · doi:10.1111/j.1469-7793.2000.00695.x
What was done
Twenty-three healthy young male and female volunteers took part in a 9-day laboratory protocol assessing circadian pacemaker sensitivity to evening light. Participants received a single 6.5-hour experimental light exposure during the early biological night across a range of illuminance levels. Endogenous circadian phase changes in the melatonin rhythm and acute plasma melatonin suppression were measured.
What was found
Both phase-delay resetting and acute melatonin suppression followed a non-linear logistic dose-response curve. Approximately half of the maximal phase-delaying response observed at ~9,000 lux was achieved with dim room light of approximately 100 lux (just over 1% of bright light intensity). The same proportion applied to acute plasma melatonin suppression.
Why it matters
Ordinary dim-to-moderate indoor room light in the evening can substantially shift the human circadian clock and suppress melatonin, meaning daily lighting habits strongly influence circadian entrainment.
Limits
Small sample size (n = 23) restricted to healthy young adults. Exact phase shift magnitudes (in hours) and statistical confidence intervals were omitted from the abstract. A continuous 6.5-hour light exposure does not fully replicate typical real-world intermittent light exposure.
Cited by
- contradicts Dimming an incandescent or halogen light source in the evening will not suppress melatonin levels or disrupt circadian rhythms.