In addition to the cold, dark weather (thanks, Ohio) and the fact that winter break is now over (sigh), my school has midterm exams, report cards, and parent conferences all in January.
Even if your exams happened in December and you have the fresh glow of a new semester to keep you inspired, the beginning of a new term can mean a lot of work, a lot of maintaining or building classroom norms, and a lot of energy to get back into the work/life groove.