Human phase response curve to a 1 h pulse of bright white light.
Level 2 - randomized trial
Individual randomized controlled laboratory trial.
PubMed 22547633 · doi:10.1113/jphysiol.2012.227892
What was done
Thirty-six participants were randomized to a 1-hour exposure of either bright white light (~8000 lux, n=18) or dim background light (<3 lux, n=18) scheduled across 1 of 18 circadian phases. Participants completed constant routine procedures in dim light (<3 lux) before and after exposure to assess circadian phase via dim light melatonin onset (DLMO). Phase shifts were calculated as the difference in DLMO timing between pre- and post-stimulus routines and compared with historical data from 6.7-hour light exposures.
What was found
A 1-hour pulse of bright white light induced a Type 1 phase response curve (PRC) with a fitted peak-to-trough amplitude of 2.20 hours, whereas no discernible PRC was observed with <3 lux dim background light. The fitted peak-to-trough amplitude of the 1-hour bright light PRC was ~40% of that observed for the 6.7-hour PRC, despite representing only 15% of the exposure duration.
Why it matters
This study shows that a brief 1-hour bright light pulse produces substantial circadian phase resetting, confirming a non-linear duration-response relationship where short light exposures provide a disproportionate resetting effect.
Limits
The sample size is small with only 18 participants per group distributed across 18 distinct circadian phases (effectively n=1 per phase). Comparisons to 6.7-hour exposures relied on historical data rather than a concurrent control group. Participant demographics, baseline sleep parameters, and statistical variance measures are not reported in the abstract.