Can Morning Light Phase Advance Human Melatonin Rhythms in Less Than 24 h?
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing existing literature with estimated parameters and no systematic meta-analysis.
PubMed 41832758 · doi:10.1111/jpi.70134
What was done
This narrative review synthesized studies using same-day or single-cycle protocols in humans, where morning light exposure and dim-light melatonin onset (DLMO) measurement occurred within the same 24-hour period. To compare interventions across studies, authors estimated melanopic equivalent daylight illuminance (mEDI) values and assessed their relationship with the magnitude of observed circadian phase advances.
What was found
Light administered shortly after morning waking was associated with modest same-day phase advances of 10 to 30 minutes, especially when bright, blue-enriched, or applied for more than 1 hour. A statistical trend toward a positive correlation was observed between estimated mEDI and the magnitude of the phase advance (r = 0.51, p = 0.06).
Why it matters
Confirming that circadian phase advances can occur within the same 24-hour cycle could shorten circadian testing protocols and accelerate clinical interventions for circadian rhythm sleep-wake disorders.
Limits
The review is narrative rather than systematic, and the abstract does not report the total number of studies or participants evaluated. Across-study comparisons relied on estimated rather than directly measured mEDI values, the dose-response correlation did not reach statistical significance (p = 0.06), and the underlying effect remains incompletely characterized.