5 hours of sleep is plenty - don't worry about that. Well let menrephrase that - if you usually sleep like 7-8 hours and you got 5 one night - don't worry a bit about it. It's not a big deal, it's not even a little deal. Even if you got 5 hours of sleep everynight you'd be fine. Butif you arefeeling sluggish turn up the head phones and eat a powerbar or snickers bar before working out (snickers only if you are not on a strict diet and don't need to cut any weight).
Please every one ignore comments like this ^^^^^
Anyway, there is a reason insomniacs supplement melatonin. It is because either their brains are not producing enough of it, or they tend to sleep later and awaken later in the day. Our brains produce melatonin at night, which is why sleep has always been associated as a nocturnal activity. Supplementing melatonin assures that you will be able to sleep regardless of the time of day, and to, more often than not, get a full 8 hours of peaceful sleep (although at first it does intensify your dreams, but this side effect fades in time). Of course, covering the windows to ensure proper darkness helps as well.