If you wake up after 5 or 6 hours well-rested, that's all you needed.

If you feel like shit when you awake, and yet can't go back to sleep to catch up on the deficit, then you may have some variety of mood-affective disorder. The excess adrenaline makes and keeps you awake as soon as you rested a little bit in the first 5-6 hours. It also makes you dehydrated, and longer-term it forms a vicious cycle due to accumulated sleep deficit. This condition requires attention of a medical professional, which I am not.

I'm... not sure. Sometimes I have trouble falling asleep (insomnia), although SOME of that can probably be attributed to late-night device usage (I use Flux, but the iPhone won't have "time-based screen yellowing" until the next iOS release.) My girlfriend, who sleeps in the same bed as me, also gets up around 6, and sometimes that wakes me (not most of the time, fortunately).

Seriously. I sleep like a baby and often show 30+ times restless.