I don't know how true this is, but it's something I've found true for myself... I've read (or heard) that your sleep cycle is 90 minutes down to deep sleep and back, so it's best to get up after some multiple of 90 minutes because that's when you'll have cycled back to the lightest sleep.

So typically I either get 6 hours or 7.5 hours of sleep a night, though if I ended up getting 3 or 4.5 one night I'll sometimes try to get 9 the next night. If I can't get at least 3 hours of sleep, I just stay up.

So it's possible the problem with 8 hours is that you're trying to wake up in the middle of a cycle and thus the sluggishness. Or maybe I'm just spreading pseudo-science.