The evidence for medium- to long-term sleep debt is sketchy at best. Certainly in the short term (1-3 days) if you are deprived of sleep, or if you're chronically short on sleep, there's all sorts of cognitive and physiological effects.

But after over longer terms of a week or more, after returning to a normal sleep schedule, it's more complicated. Much of the 'overtired' feeling and cognitive effects come from being deprived of slow-wave sleep (SWS), which you get in the deepest phases of sleep. It's in this phase of sleep that motor skills are consolidated, and it seems to be the 'physically restorative' part of sleep. This is opposed to REM sleep, which you get in the lighter phases of sleep and is where you dream and where memories are consolidated. Unlike SWS, there are very few side effects to being deprived of REM sleep. In a typical eight-hour sleep there will be several cycles of sleep, totalling about four to five hours out of eight spent in SWS, and the remainder in REM and in transition between states.

As for sleep debt and why it doesn't really exist beyond the short term, it comes down to what sleep researchers call sleep architecture, or how the cycles and phases of sleep are arranged through a night's sleep. For example, there's typically a bias towards getting more SWS in the first half of the night, and more REM sleep in the second. Here's what a typical night's sleep looks like (Stages 3 and 4 are SWS sleep):

http://en.wikipedia.org/wiki/Sleep_architecture#mediaviewer/File:Sleep_Hypnogram.svg

Your sleep architecture changes under sleep deprivation. If you are selectively deprived of SWS one night (e.g. by an evil sleep researcher waking you up any time you enter SWS), you will spend more of the next night in SWS to compensate. If your sleep is reduced gradually over time, your body will sacrifice REM sleep to maintain the same amount of SWS -- about four to five hours a night. As I said before, not getting much REM sleep has few side effects.

This is why you are able to sleep only five hours a night and still feel fine: your body simply adjusted to spend very little time in SWS. If I could guess, I'd say you probably have very few dreams anymore, maybe just one occasionally, shortly before waking up? I can't explain why this helped your insomnia or headaches, because I don't know much about how an insomniac differs from a typical person. But it's not to do with sleep debt.