Change jobs

I used to work in a large office and between the lack of fresh air/circulation and the "white noise" of the computers and xerox machines, I'd be quite sleepy. If you have a desk job, think about getting a small plant to boost the oxygen near you. Many people say they breathe better when they sit near plants. Also a crisp wintergreen or spearmint candy can give you a little pick me up without too many calories, and remember to drink plenty of water.

Red Bull.
Ginko Biloba.
B12 vitamins.
Exercise.
A short nap before work.
And try to make your job fun (if possible).

its the weirdest thing isn't it? I am the same way...actually now i'm just tired all the time.
I've even gotten sick and as soon as i go home i feel better...that's sad. Well, it just means that work isn't interesting you and that it's draining you. Ask for a raise...that ought to give you a boost.

Maybe move around more! It always helps me...

AnswerBag!

Adjust the temperature, or your clothes. Or grab the occasional nap
http://news.yahoo.com/.../napsmayboostmemory

Splash cold water on your face (and do some stretching exercises while in the wash room).

Find a sexy co-worker slip under her desk and give her face!! That will wake you up

It can be chemicals used at work to clean etc.,
or everyone is bored there and you are picking it up!