Modern life style differs significantly from these early days. The windows have heavy drapes, so most of us do not see the sunlight except when we peek outside. In the evening, many of us stay awake to watch the late night shows. The problem is that our system needs time to sleep. Studies on animals have shown that they have definite patterns they follow every day depending on the season.
Many birds migrate to south for the winter.
But, come spring, nature become very lively. The birds return from the south. The trees starts the new growth. Many animals are found to time the events in their lives depending on the season, so that the functions can be accomplished at the most effective way. For example, lambs are born only in the spring when there is plentiful of food for the mother to nurse the newborn.