[Matajic]

What a coincidence, I'm tired of being sick :-p

But seriously, you probably need to get more sleep, get a better night's sleep, etc. Look for irregularities in your sleeping routine, they might be a cause.
Or your diet could be another cause. Eat balanced meals, and a vitamin is a good idea, too. Go heavy on carbs, since they are the body's primary fuel source and all.