The ancient sages understood that what happens in the natural world - "out there" - also happens within. Whether conscious of it or not, seasonal changes have a profound impact on one's state of being; emotionally, mentally & physically.
We must take special care of ourselves during the energetic transitions between the seasons. We must sleep, eat and move in accordance with the rhythms of a given season. The Classics teach that longevity & vitality are granted to those who obey the natural laws.