Here are some surprising things that can wear you out, courtesy of IVillage.com:

Energy buster #1: All work and no play. Being a responsible adult is exhausting. All those routines you do day in and day out can get tedious. And your body registers boredom as tiredness. According to Dr. Alice Domar, director of The Mind-Body Center for Women's Health in Boston, the constant push to be efficient and productive drains you both physically and emotionally. So make sure you give your brain some downtime. Flip through a magazine, or call a friend. These mini-breaks will take the dreariness out of your routine.

Energy buster #2: Overdoing exercise. Working out every day may not be a good energy strategy, especially if you're a beginner or you're trying to get back in shape. That's the word from Richard Cotton, chief exercise physiologist for the American Council on Exercise. He says the body needs time to recover after exercise - to replenish energy and repair muscles. If you don't respect that and you just keep working out all the time, you'll wind up in a constant cycle of fatigue.
So take a day between workouts and DON'T over exert yourself.

And one last thing that drains your energy: Too much noise. According to psychologist Arline Bronzaft, noise is stress. When there's too much of it, your pulse rate and blood pressure increase and your adrenaline surges. This puts added stress on the nervous system and eventually depletes your energy. Also, you have to work harder to complete tasks because you're trying to ignore the sounds around you. That's also physically exhausting. So, get ear plugs, head phones, or even a white noise machine   anything that'll mask the sounds that distract you from your work. Your energy level depends on it.