This one ties in with the weather and general wintertime blues. Short days and cold weather are more likely to inspire you to curl up with a warm fire and a hot toddy than to head outside and pound the pavement. Even if you don't count a drop in exercise, there's a marked difference in the activity level between winter months and more moderate ones. Spring and summer will likely find you tackling home projects, doing yard work, maybe hitting the pool or taking part in an organized team sport. Even going to the park and tossing the Frisbee is going to burn a lot more calories than roasting chestnuts on an open fire. So even if you do manage to brave the cold and hit the gym, or you're an active person in warm weather, you'll notice a drop off in your daily activity level. This is why even fit people can gain a little weight over the holidays.