Effects of Melanopic Equivalent Daylight Illuminance on Sleep Regulation and Chronotype-Specific Responses in Young Adults

ObjectivesLight is a key regulator of the human circadian system, yet conventional photopic illuminance does not reflect the spectral sensitivity of intrinsically photosensitive retinal ganglion cells (ipRGCs). Melanopic equivalent daylight illuminance (EDI) provides a more biologically relevant metric. We examined whether melanopic EDI better predicts real-world sleep outcomes than photopic illuminance and whether these associations differ by chronotype. MethodsFifty-nine young adults wore actigraphs for seven days to monitor light exposure and sleep. Light exposure was quantified as photopic illuminance and melanopic EDI. Chronotype was classified using the Morningness-Eveningness Questionnaire (MEQ) as non-evening type (nET) and evening type (ET). Hierarchical regression assessed the added predictive value of melanopic EDI, and linear mixed-effects models examined temporal associations and chronotype effects. ResultsMelanopic EDI improved the prediction of sleep outcomes across all time windows. The largest delta R2 occurred for sleep quality in the afternoon (1.55%) and at night (1.00%), deep sleep at night (0.93%), and fragmentation in the afternoon (0.51%). Nighttime exposure (01:00-03:00) was associated with poorer sleep quality in both chronotypes (nET: p=0.018 (01:00), p=0.015 (03:00); ET: p=0.043 (01:00)). Morning exposure (10:00) improved sleep quality (p=0.002) and reduced sleep fragmentation (p=0.031) in nET, whereas evening exposure (18:00-24:00) was associated with lower sleep quality (p=0.002) and greater sleep fragmentation (p=0.027) in ET. ConclusionsMelanopic EDI is more sensitive to sleep than photopic illuminance. Morning light benefited nET, whereas evening light disrupted sleep in ET, supporting melanopic metrics and chronotype-based light strategies to improve sleep health. Statement of SignificanceLight, especially short-wavelength blue light, exerts non-visual effects on human physiology through ipRGCs that synchronize the circadian system. However, conventional light metrics, such as photopic illuminance, do not capture these spectral characteristics, limiting their ability to predict physiological outcomes. Using real- world data, this study demonstrates that melanopic EDI is a more sensitive predictor of sleep quality and structure than photopic illuminance. Temporal and chronotype-specific analyses showed that morning melanopic light improves sleep in nET, whereas evening exposure disrupts sleep in ET. These findings address a gap by demonstrating the ecological validity and chronotype-dependent impact of melanopic-sensitive light metrics. These results underscore the need to incorporate melanopic metrics and chronotype considerations into personalized sleep hygiene strategies, clinical recommendations, and public health guidelines, and point toward developing targeted interventions that leverage spectral light properties to optimize circadian and sleep health.