That's interesting, I actually used to find something similar. I have chronic fatigue syndrome (chronic = persistent, fatigue = tired, syndrome = god knows why). The upshot of this is when I am under any amount of stress I become very prone to poor / low sleep. For example I have a deadline coming up and I am writing this at 3am.

During particularly bad periods I did find that hours awake from midnight to ~5am were both very productive and very happy. No idea what that's about. Fortunately I don't drive so sleep deprivation is not such a dangerous issue.