Can Morning Light Phase Advance Human Melatonin Rhythms in Less Than 24 h?

Light is the primary cue that synchronises the human circadian system to the 24-h day, advancing or delaying circadian rhythms depending on its timing. While it is known that morning light induces phase advances, most studies assess the timing of dim-light melatonin onset (DLMO) in the subsequent circadian cycle, over 24 h after the light intervention. However, it is unclear whether these phase advances occur within the same circadian cycle as the light intervention or the next one. This narrative review addresses the question of whether morning light can phase-advance human melatonin rhythms in less than 24 h. To answer this question, we review studies that use same-day or single-cycle protocols, in which light exposure and post-intervention DLMO assessment occur within the same 24-h period. To compare light interventions across studies, melanopic equivalent daylight illuminance (mEDI) values were estimated and related to the magnitude of the observed phase advance. The reviewed research suggests that modest phase advances of 10-30 min can be achieved within the same circadian cycle if light is delivered shortly after waking up in the morning. This is particularly effective if the light is bright, blue-enriched, or if it is delivered for a long time (over 1 h). There was a statistical trend (r = 0.51, p = 0.06) towards a positive association between the mEDI of the light intervention and the magnitude of the phase advance. Overall, same-day phase-advances seem possible but not well characterised, and more targeted work is needed to determine whether morning light can phase-advance human melatonin rhythms in less than 24 h. If this is confirmed, the length of circadian protocols could be reduced, accelerating the clinical use of treatments for circadian rhythms sleep-wake disorder.