Everyone is different, some people function better on the recommended amount of sleep, some function better on more and then there's the (apparantly) smaller group which I fall into that function better on less sleep.

I tried the whole 8hrs sleep schedule and I just could not keep that going for long even though it was recommended by my pdoc. It made me feel sluggish and wanting more sleep. The only time I can and do sleep longer is when I'm depressed.

So I went back to my normal which is between 4 and 6. I function just fine on this schedule whether I'm working or not. When I'm hypomanic I sleep between 0 and 2 hrs per night, so I don't let my 4hr nights worry me.

Sudden changes in sleeping patterns can definitely be an indicator of a mood shift, so what you should be looking out for is additional signs / symptoms which will help you identify if you're heading up.