I find that it becomes self-perpetuating if you let it get out of control, but at the onset I always have a trace of mania that makes me not want to sleep. Elsa referred to insomnia. To me, that's, "Oh god, I'm so tired, why can't I just sleep?!" At the onset of hypomania my sleeplessness is just wakefulness. No desire to get in bed, no desire to rest yet. Not needed. But it is needed, eventually. You can go a while, but if you don't catch it, and if you don't get enough sleep, you get more manic. Tricky tricky stuff.