Could be a symptom of poor stimulation regulation. I know that's the problem in my case. if I have a phone, screen, or even a book in front of me I can stay up until tomorrow. But, if I shut it all down (roughly 30 to 60 mins) before I actually plan to try and sleep it absolutely causes the tiredness to kick in.

I will say there are varying degrees of stimulation. Computer use > TV > Book reading > Music > silence and boredom. At least that is the order in which I shutdown on the nights I have the motivation to do so.