7.5 a night at the moment, I do the whole 90-minute sleep cycle thing too.
The theory behind it is that humans sleep in cycles of an hour and a half, moving from light sleep, down into deep sleep, then back up into REM sleep where we're at the best point to wake up (REM or just after in light sleep before going back down). That's why 8 hours doesn't really work very well - odds are most people are back down into semi-deep or even totally deep sleep by then.
Everyone sleeps at a slightly different rate, though - I find mine are a little less than 90 minutes, because I can get by on about 7 hours and twenty, but with 7 hours and forty I wake up groggy. So there's experimentation there too.
Another tip for waking up without grogginess is to wake up to light instead of an alarm - sleep with your curtains open. Doesn't work too well in the British winter, though :P