 Dr. Christina Lee with Kaiser Permanente says seasonal depression is more commonly thought of as a winter phenomenon, but 10% of people with seasonal depression issues are actually more affected in weather like this. It's linked to an overabundance of sunlight and then other environmental stressors like increased heat, increased humidity, and high pollen levels. Dr. Lee says summertime sad could become even more common as global temperatures get more severe. Heat and excess light can induce fatigue and also mess with circadian rhythms for sleep.