Are you on any meds besides lithium? Some of them can be quite sedating. I've heard that seroquel is pretty bad. I'm on tegretol, and I've noticed an increase in fatigue while on it. The 3rd day I took it, I slept 17 hours (not all at once)! Thankfully it's gotten better...

It could be the illness. Depression definitely causes fatigue; during my seasonal fall episode last year, I was worried that I had narcolepsy, because I also had sleep paralysis. I wasn't dx-ed then, so that fatigue was purely depression-related. Do you have any other depressive symptoms right now?

Undiagnosed physical illness is yet another possible cause. Annie mentioned sleep apnea, I mentioned narcolepsy...but there are other possible causes. Some of them are pretty scary-cancer and MS are among the diseases that can be energy-sapping. I'm not trying to cause you to freak out- most likely, you don't have anything life-threatening or life-altering other than bipolar. But it's always better to be safe than sorry. So schedule an appointment with your primary care doc.

I hope that you can find a solution for this soon; I understand how frustrating it can be!