Daily light exposure differences between extreme chronotypes persist after alignment to habitual sleep timing

Purpose: Morning and evening chronotypes may receive light at different clock times, thereby providing differently timed signals to the circadian pacemaker. Prior studies have suggested phase-specific exposure patterns that could counter further advances in early types and delays in late types. We examined whether photoperiod modifies these exposure patterns in a way that could reasonably counteract timing extremes. Methods: We analysed near-eye melanopic equivalent daylight illuminance (EDI) in 140 participants from an international cohort across seven countries (778 participant-days; 35,949 half-hour observations). We compared four quartiles of midsleep on free days corrected for sleep debt (MSFsc) in clock time and relative to MSFsc using cyclic generalized additive mixed models, accounting for day type, site, photoperiod and individual patterns. In exploratory analyses, we tested moderation of these differences by daylength and used phase-response-curve (PRC) weighting as a conceptual comparison of exposure timing. Results: The modelled extreme evening-type/morning-type exposure ratios averaged 0.16 (95% CI 0.06-0.40) 9.6-11.3 hours before MSFsc (pre-MSFsc) and 14.0 (5.6-34.8) 0.9-4.4 hours after MSFsc (post-MSFsc). The models performed similarly well by ordinary in-sample R2 (0.795 for clock time; 0.794 for MSFsc-relative time). Pattern differences were broadly similar on work and free days, with the pre-MSFsc contrast descriptively attenuated on free days. The post-MSFsc ratio increased by a factor of 2.59 (95% CI 1.06-6.34) between civil photoperiods one hour below and above each site's average photoperiod, with no significant moderation of the pre-MSFsc contrast. Exploratory weighting by the published PRC gave higher advance-weighted point estimates in extreme evening than extreme morning types; the group difference was not statistically significant. Conclusions: Extreme chronotypes differ in daily exposure both in clock time and relative to MSFsc. The established questionnaire-based MSFsc measure showed significant post-MSFsc daylength moderation. The exposure patterns and conceptual PRC comparison suggest a possible pathway linking daylight availability to entrainment in extreme chronotypes.