Work struggles

Is anyone else finding it impossible to get thru a full day at work? I work full time in accounting and I have been having the worst time. I'm so tired and nauseated all day that I find myself leaving early too often because I just can't seem to make it to 5 o'clock. Any tips or suggestions? My boss does know what is going on and is very supportive, I just hate feeling like a 'flake' even though I have the best reason.