Seasonal variation of the pupil light reflex

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PurposeVariation of retinal light sensitivity between winter and summer seasons may be a marker for seasonal affective disorder and the basis for response to phototherapy. As the pupil is a marker of retinal light sensitivity and shows a 24‐hour variation that reflects circadian rhythm, we used the pupil as proxy to assess light sensitivity of rods, cones and melanopsin as a function of seasonal light in healthy adults.Methods37 adults were tested during the short (January‐February) and long (July‐August) annual photoperiods. All subjects completed standardized questionnaires of seasonality and sleep. Pupil responses to blue (470 nm) and red (622 nm) light were recorded under dark and light adapted conditions using an automated pupillometer. Main outcome parameters were maximal contraction amplitude and post‐stimulus contraction amplitude.ResultsAll but two subjects reported good sleep quality. One subject was an extreme morning type. Twelve subjects reported a global seasonal variation in sleep, mood and appetite but without dysfunction or disability. Baseline pupil size of light adapted eyes was greater in summer compared to winter. Maximal contraction amplitudes to scotopic and photopic light stimuli were greater in summer. The post‐stimulus pupil contraction was also greater in summer, but only after light adaptation.ConclusionsBaseline pupil size and pupil responses derived from rods, cones and melanopsin photoreception were greater in summer in healthy adults. This seemingly greater retinal sensitivity to light when the photoperiod (daylength) is long suggests a more central basis for seasonal regulation of the pupil light reflex.