Hot days start after summer solstice. Around the longest day, the farther from the equator you live, the longer day light you get. At poles i.e; North pole or South pole the sun never sets up at this time of year.
Warm weather stimulates all living in the area. Cultivation season starts in many parts. Wild life also gets stimulated and starts breeding, also getting and storing food for cold days. So the hot weather stimulates brain to produce hormones in all living.