For me, it may mean I am sick, or about to get sick. It may mean I have been running and training especially hard. It may be because I have been traveling and missing sleep. It could be from the winter, who is motivated to go for a run totally in the dark and cold after a long day at work? I think it's a phase too, some sort of biological clock that is a function of all the above. I worry sometimes it's a symptom of something bigger.
I went to sleep at 7:30 last night, and I haven't been up past 9:30 since getting back from Germany, I'm exhausted.