I disagree that there is not an external reason for this.
It's Autumn, Summer is dieing. During Autumn,we begin to see the waning of the Sun more obviously now as the days continue to grow shorter until the Wheel of the Year spins around again to Yule. We mourn the passing of Summer; the long sunny days, the growing Earth. Autumn is a time of decay, you can smell it in the air. But Autumn is also a time to rejoice. It's a season of beauty. A time to slow down our frantic pace. A season to gather with family and friends.
You want to cocoon, crawl under blankets and sleep, watch sad movies, read books? You should make time to do those things.
In times past our ancestors slowed down, told stories around the fire, spent time with family and friends they were to busy to do during spring and summer. That is what we are meant to do.
We, as humans, are Earth based beings and as such (though we hate to admit it) we are effected by the changing of the seasons.

The getting older part doesn't help either. Just wait until you are 38 going on 40.

*hugs*