Similarly to how a lack of sunlight causes circadian rhythm disruptions, excessive sunlight can have the same effect. Too much sunlight will disrupt melatonin production, causing insomnia, sleep deprivation, and irritability. The body's circadian rhythm won't be able to correctly regulate sleep-wake cycles, and combined with the summer heat, this can result in the symptoms patients with summer MDD report.

Georgetown University

Seasonal MDD that flares up during summer months is less common than the typical winter MDD. The two types of seasonal MDD have notably different symptoms in affected patients. 

MDD with Seasonal Summer Pattern

Caijuan Xu, Weijia Wu, Danni Peng-Li, Peilin Xu, Dong Sun, Bin Wan,
Intraday weather conditions can influence self-report of depressive symptoms, Journal of Psychiatric Research, Volume 123, 2020, Pages 194-200, ISSN 0022-3956, https://doi.org/10.1016/j.jpsychires.2020.02.006.

Journal of Psychiatric Research

Summer MDD is more common among residents of equatorial countries. These countries face higher temperatures that spike during the summer, causing heat waves. These heat waves have been linked to anger and temperament issues in summer MDD patients, particularly in developing countries where air conditioning is less prevalent. 

High Heat and Seasonal Summer MDD

Excessive Sunlight and Summer MDD

- Winter MDD patients often report lethargy, feeling sad, and sleep issues, and more suicidal thoughts.
- Summer MDD patients report anger, anxiousness and insomnia. This is likely due to how different seasonal weather patterns, such as heat and excessive sunlight, impact the brain.

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