Adaptation of human pineal melatonin suppression by recent photic history.
Level 3 - non-randomized controlled study
Controlled within-subjects laboratory trial without reported randomization
PubMed 15240654 · doi:10.1210/jc.2003-032100
What was done
Eight participants were studied under controlled laboratory conditions in a within-subjects design. Baseline melatonin secretion was measured under constant routine conditions. Melatonin suppression was then evaluated during a fixed 6.5-hour light stimulus of approximately 200 lux (50 µW/cm²) under constant routine conditions following two different ~3-day photic histories during the subjective day: a dim light condition (~0.5 lux; 0.15 µW/cm²) versus a moderate light condition (~200 lux; 50 µW/cm²).
What was found
Light-induced melatonin suppression during the 6.5-hour 200-lux stimulus was significantly greater following the ~0.5 lux photic history than following the ~200 lux history. The abstract did not report exact numerical percentages, absolute concentrations, or p-values.
Why it matters
This demonstrates that the human circadian system exhibits adaptive photoreception based on recent light history. Understanding how prior light exposure alters circadian light sensitivity could help optimize lighting design and phototherapy protocols for circadian and affective disorders.
Limits
The sample size was very small (n = 8). The abstract does not specify whether the order of photic history conditions was randomized, nor does it provide participant demographic characteristics or exact quantitative suppression data.
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