I never did work it out myself. Sometimes I was fine. Other times I didn't sleep for 3 days. Was "sort of" independent of stress and so on, although higher average stress levels made it a little worse. Or maybe stress just feels worse combined with sleep deprivation. I tried various changes to my habits over the next few years, none of which made any difference -- although some were good for other reasons (e.g. getting into better shape, eating better, and so on).

Went to go see a doctor. They brushed it off, so I went to go see another doctor. They prescribed a low dose of a sleeping pill.

Problem solved forever with no noticeable side effects. I think I'm on 1/2 the pediatric dose or something. Amazing how so little of something can make such a big difference in my life. Wish I had gone to see 2 doctors earlier.