I know in the winter, if left alone, I will hibernate. When it gets dark, I'm ready to crawl in bed by 8. I tried to get up at 6:30 for a bike ride this morning, but the sun wasn't up--so I rolled back over and went back to sleep!

To counteract this--in the evening, I make an effort to turn on every light in the house--this keeps me awake until at least 9 pm. When I worked in an office, I went to the gym every day after work--that also kept me up a little later.