Naturalistic light exposure patterns in relation to medication status, mood symptoms, and chronotype

Light assists in regulating mood, and selective serotonin reuptake inhibitors (SSRIs) increase non-visual light sensitivity. It remains unclear whether light exposure patterns differ between individuals taking SSRIs compared to unmedicated individuals with no psychiatric history, or how everyday light exposure relates to mood and chronotype. This study examined objective light exposure (melanopic EDI) in relation to medication status (SSRI vs. control), mood symptoms, and chronotype. Participants (n = 76; 38 SSRIs, 38 controls) completed at least one week of field light monitoring using a wearable sensor and questionnaires assessing mood (DASS-21) and chronotype (MEQ). Overall light exposure did not differ between groups. However, when accounting for group, greater morning light exposure was associated with lower depressive and stress symptoms, and more time spent above 50 melanopic EDI was associated with fewer depressive symptoms. Greater morningness was linked to higher morning and daytime melanopic EDI, more time in bright light (>50 and >250 melanopic EDI), and differences in light regularity. These findings show that light exposure, particularly its timing and amount, relates to mood and chronotype, regardless of SSRI use. Future research targeting light behaviour may offer an accessible, cost-effective strategy for improving mood in both clinical and non-clinical populations.