Morning-evening differences in the post-illumination pupillary response are associated with seasonal mood changes

Changes in light availability during the winter may lead to individuals experiencing seasonal changes in mood, sleep and appetite, and in the most severely affected, seasonal affective disorder (SAD). By virtue of later activity schedules and less morning light exposure, people with evening chronotype may show higher levels of seasonal mood variation. Light information is transmitted from the eye to the brain through intrinsically photosensitive retinal ganglion cells (iPRGC), neurons that help coordinate circadian and seasonal rhythms. The post-illumination pupillary response (PIPR) is an observable physiological proxy measure of iPRGC activity in humans. Past work has shown that sensitivity of iPRGC to light measured by PIPR is associated with chronotype and seasonal changes in mood, sleep and behavior. However, the sensitivity of IPRGC to light may itself vary across the day, and how these changes in light sensitivity relate to seasonal mood variation and chronotype remains poorly unstudied. To estimate morning-evening differences in iPRGC light sensitivity, we used a pupillometer to measure PIPR following blue light pulses in the morning and evening 12 h apart in 20 community volunteers. Participant chronotype was assessed using the Basic Language Morningness Scale. Seasonal mood variation was assessed using the Seasonal Pattern Assessment Questionnaire. PIPR was significantly larger in the evening compared to the morning. After adjusting for age and sex, morning PIPR was associated with seasonal mood variation. We did not find any association between PIPR and chronotype. Our work supports the hypothesis that iPRGC sensitivity influences seasonal mood variation and that PIPR shows morning-evening differences. Morning light may be relevant to the development of SAD.