He observed that dark night and daylight alternate daily and the inhabitants of the world responded to this rhythm. The long cold winter is followed by a hot season. Each dominates the world in turn that you could depend on. The two extremes of light and dark and hot and cold alternate periodically but not identically. The world responds to each season.
Plants become active in the warm season and rest under ground in the cold season.
Activity in animals and people respond to the daily and yearly cycles. Activity and sleep harmonize with the daily cycle of day and night. In the summer when it's hot and the days are long, humans tend to be more active and in the winter when the days are cold and the nights are long we tend to pursue mental activities. Exceptions to the rule exist some people and animals are active at night. But both extremes are necessary to life.