Late evening room light and sleep restriction reduces the ability of bright morning light to phase advance adolescents' circadian clocks.
Level 3 - non-randomized controlled study
Non-randomized or unstated-randomization controlled experimental trial comparing four parallel bedtime delay groups.
PubMed 41730935 · doi:10.1038/s41598-026-37985-y
What was done
Adolescents aged 14–17 years (n = 40) were assigned to four sleep opportunity conditions created by delaying bedtime: 0 h delay (10 h sleep opportunity, n = 11), 1.5 h delay (8.5 h sleep, n = 9), 3 h delay (7 h sleep, n = 12), or 4.5 h delay (5.5 h sleep, n = 8). Participants received 90 minutes of morning bright light on 3 consecutive mornings while the sleep/dark episode gradually shifted earlier.
What was found
Circadian phase shift magnitude changed in a step-wise manner across groups: - 10-h sleep group: phase advanced by 2.0 ± 1.1 h - 8.5-h sleep group: phase advanced by 0.5 ± 0.6 h - 7-h sleep group: phase delayed by 0.8 ± 1.1 h - 5.5-h sleep group: phase delayed by 2.6 ± 1.2 h
Why it matters
Morning bright light is commonly used to advance delayed circadian rhythms in adolescents, but staying awake late under room light can eliminate this benefit and induce net circadian delays. Effective circadian phototherapy protocols must manage evening light exposure alongside morning light administration.
Limits
Group sample sizes were small (8 to 12 participants per arm). The abstract does not report whether allocation was randomized, nor does it specify the lux levels of evening or morning light. Evening light duration and sleep restriction were co-manipulated, so their independent effects cannot be separated from the abstract data.
Cited by
- supports Getting bright sunlight in the morning for 10 to 30 minutes resets your circadian rhythm.