Even short of polyphasic sleep, try getting between 6 and 7 hours of sleep, make yourself get up and see how that does. Often we get "sleep inertia" where your body keeps just pumping the melatonin into your system when it really doesn't need the rest. I know that aside from a polyphasic schedule, I can sleep for 8-10 hours, but anytime I do it I feel groggy all day - melatonin in the bloodstream - this has nothing to do with having enough sleep or sleep debt.