Its expected - that little thing growing inside absolutely zaps you of all your energy - its crazy!
I was drifting off at my desk at work too so I know how it is. Just take frequent naps (even after work if you can) and get plenty of rest when you go to bed at night - and whenever you have days off of work. There's not a whole lot you can do other than fit in sleep whenever possible.
Don't rely on coffee or energy drinks either. And if you exercised before you got pregnant, keep up with it to a lesser extent, but don't start if you didn't before. Fresh air while taking walks helps too, even if its after supper or on a lunch break.
It'll pass within a few weeks! You won't feel as tired around the 2nd trimester.