High sensitivity and interindividual variability in the response of the human circadian system to evening light.
Level 3 - non-randomized controlled study
Controlled within-subjects experimental physiological study
PubMed 31138694 · doi:10.1073/pnas.1901824116
What was done
Researchers measured dose-response curves for light-induced melatonin suppression in 55 healthy young adults aged 18 to 30 using a within-subjects laboratory protocol. Participants were exposed during the evening for 5 hours to a dim control condition (<1 lux) and experimental light levels ranging from 10 to 2,000 lux. Group and individual effective doses for 50% melatonin suppression (ED50) and changes in apparent melatonin onset timing were calculated.
What was found
The group-level fitted ED50 for melatonin suppression was 24.60 lux. Light exposures of 10, 30, and 50 lux delayed apparent melatonin onset by 22, 77, and 109 minutes, respectively. Individual ED50 values varied by more than an order of magnitude, ranging from 6 lux in the most sensitive participant to 350 lux in the least sensitive participant, with a coefficient of variation of 26%.
Why it matters
Typical indoor evening light levels are sufficient to cause substantial circadian delay and melatonin suppression in humans. Furthermore, large individual variations in light sensitivity mean that the same indoor lighting environment will affect different people to drastically different degrees.
Limits
The sample was restricted to healthy young adults aged 18 to 30, limiting generalizability to older populations or clinical cohorts. The abstract does not report light spectrum characteristics, prior light exposure history, or direct downstream clinical endpoints such as sleep architecture or health outcomes.
Cited by
- contradicts Dimming an incandescent or halogen light source in the evening will not suppress melatonin levels or disrupt circadian rhythms.