A phase response curve to single bright light pulses in human subjects.
Level 2 - randomized trial
Controlled human experimental trial evaluating physiological phase shifts to timed interventions
PubMed 12717008 · doi:10.1113/jphysiol.2003.040477
What was done
Twenty-one healthy, entrained subjects underwent pre- and post-stimulus constant routines in dim light (approximately 2-7 lx) with continuous wakefulness in a semi-recumbent posture. A 6.7 h bright light exposure consisting of alternating 6 min fixed gaze (approximately 10 000 lx) and free gaze (approximately 5000-9000 lx) was scheduled across different circadian phases. Phase shifts were assessed by measuring plasma melatonin onset, offset, and midpoint profiles.
What was found
The phase response curve for the midpoint of the melatonin rhythm showed a type 1 curve with a significant peak-to-trough amplitude of 5.02 h. Light centered before the core body temperature minimum caused phase delays, light after caused phase advances, and light at the critical phase produced no shift. No prolonged dead zone of photic insensitivity occurred during the subjective day. Melatonin onsets showed larger phase shifts than melatonin offsets.
Why it matters
The study precisely characterizes the human circadian phase response curve to light under controlled constant-routine conditions, showing that light sensitivity persists throughout the subjective day without an extended dead zone.
Limits
The sample size was small with 21 participants, and all were healthy entrained individuals studied under artificial laboratory conditions. The 6.7-hour high-intensity light stimulus does not represent natural or typical real-world light exposures.
Cited by
- context Exposure to approximately 10,000 lux of bright blue-spectrum light can shift the human circadian rhythm by about 8 hours.