A key symptom of depression is lack of willpower - depressives don't normally have the willpower not to sleep.

For me personally, and I suspect also for a significant number of other people, it takes willpower to go to sleep as well as to wake up early enough. In the morning, the path of least resistance for me is to sleep in, but in the evening, it is to do something fun until I'm overcome with overwhelming sleepiness, which won't happen until it's far too late to maintain a normal sleeping schedule. Therefore, if I were completely deprived of willpower, my "days" would quickly degenerate into cycles of much more than 24 hours, falling asleep as well as waking up at a much later hour each time.

Now, the incentive to wake up early enough (so as not to miss work etc.) is usually much stronger than the incentive to go to bed early enough, which is maintained only by the much milder and more distant threat of feeling sleepy and lousy next day. So a moderate crisis of willpower will have the effect of making me chronically sleep-deprived, since I'll still muster the willpower to get up for work, but not the willpower to go to bed instead of wasting time until the wee hours.

(This is exacerbated by the fact that when I'm sleep-deprived, I tend to feel lousy and wanting to doze off through the day, but then in the evening I suddenly start feeling perfectly OK and not wanting to sleep at all.)