The effects of prior light history on the suppression of melatonin by light in humans.
Level 3 - non-randomized controlled study
Within-subject crossover experimental study without stated randomization in the abstract
PubMed 12390501 · doi:10.1034/j.1600-079x.2002.01885.x
What was done
Twelve subjects completed two 1-week light exposure protocols: a bright week (4.3 ± 0.4 hours/day of bright light outdoors and via light boxes) and a dim week (wearing ~2% transmission dark goggles outdoors, spending 1.4 ± 0.9 hours/day outside). Saliva samples were collected every 30 minutes for 7 hours under dim light (<15 lux) on two consecutive nights (baseline and test night) at each week's end. On test nights, 500 lux was administered for 3 hours during the collection period to assess melatonin suppression.
What was found
Melatonin suppression was significantly greater following the dim week than the bright week (suppressed to 53% versus 41% of baseline values, P < 0.05). Large individual differences occurred, with the response difference between the two conditions being pronounced in 7 of the 12 subjects.
Why it matters
This study provides evidence that the human circadian system dynamically adjusts its light sensitivity based on recent photic history, meaning prior daytime light restriction increases susceptibility to nocturnal light-induced melatonin suppression.
Limits
The sample size was very small (n = 12), and the abstract does not report whether condition order was randomized. A noticeable response difference was absent in 5 of the 12 subjects, and demographic details such as age and sex were not reported.
Cited by
- partial Evening light exposure of 10 to 30 minutes partially prevents the suppression of melatonin caused by subsequent bright artificial light exposure at night.