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• Perinatal photoperiod associations with bipolar and depression disorders are novel. • Associations were observed in the extensive UK Biobank database. • Findings are compatible with season of birth and depression studies. • Perinatal photoperiod may reconcile inconsistencies in season of birth studies. • Findings build on previous research by ourselves and other groups. • More extreme photoperiods need to be accounted for. • If associations are true, the door is opened to simple preventive strategies. Season-of-birth associations with psychiatric disorders point to environmental (co-)aetiological factors such as natural photoperiod that, if clarified, may allow interventions toward prevention. We systematically reviewed the literature concerning season-of-birth and bipolar disorder and depression and explored associations between the perinatal natural photoperiod and these outcomes in a cross-sectional analysis of the UK Biobank database. We used mean daily photoperiod and relative photoperiod range (relative to the mean) in the 3rd trimester and, separately, in the first 3 months post birth as metrics. From review, increased risk of depression with late spring birth is compatible with increased odds of probable single episode-, probable recurrent-, and diagnosed depression (OR 2.85 95 %CI 1.6–5.08, OR 2.20 95 %CI 1.57–3.1, and OR 1.48 95 %CI 1.11–1.97, respectively) with increasing 3rd trimester relative photoperiod range for participants who experienced relatively non-extreme daily photoperiods. Risk of bipolar disorder with winter-spring birth contrasted with no consistent patterns of perinatal photoperiod metric associations with bipolar disorder in the UK Biobank. As natural photoperiod varies by both time-of-year and latitude, perinatal natural photoperiods (and a hypothesized mechanism of action via the circadian timing system and/or serotonergic circuitry associated with the dorsal raphe nucleus) may reconcile inconsistencies in season-of-birth associations. Further studies are warranted. [ABSTRACT FROM AUTHOR]