A single day of natural daylight exposure and physical activity does not conclusively change circadian phase: A Bayesian crossover experiment

A single 40-h visit with 8 h of natural daylight (1196 lx) and optional hiking does not yield conclusive circadian phase shifts in 41 healthy adults.

• Why it matters: Light is the main daily cue for human circadian timing, but the independent and combined roles of daylight and physical activity under naturalistic conditions remain hard to disentangle.
• What they did: In a counterbalanced mixed-design crossover, 41 adults completed up to three 40-h visits with natural daylight, amber-filtered daylight, or indoor light, plus hiking vs sedentary; Bayesian hierarchical models predicted change in dim-light melatonin onset using photopic illuminance (ActTrust 2) and median heart rate reserve (HRR).
• The result: Per SD increase in log illuminance and median HRR, estimated DLMO changes were -4.22 min (95% HDI [-10.93, 2.22]; BF = 0.70) and -6.48 min (95% HDI [-13.79, 1.27]; BF = 1.56), with evidence inconclusive for both predictors and their interaction.

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