Exhaust your body - put on techno and dance, work out, do something physical that will get your blood pumping... it's what helps me when I'm in that state.
Just a note to the non-ADHD lurkers (we love you guys) my understanding is that this is an ADHD specific thing where we need the come down from stimulation to "switch off" sometimes - I think it's probably awful advice for neutotypical folks and will just flood you with endorphins that keep you awake.
Also, as always, ADHD isn't monolithic and your mileage may vary.
Ah wait this need for a "come down" from stimulation might explain a lot of things for me.
If I'm coming down from a long day/walk/etc. the tiredness hits me, but if I don't go to bed and fall asleep fast enough, even if I'm relaxing, then I will no longer be sleepy and will struggle to fall asleep.
Definitely the absolute worst - your brain is constantly being fed interesting things... I've found Civ and similar games bringing me to dawn without realizing it.
The only thing I've found that helps is feeling like I achieved something worthwhile in my day. So after 6+ hours of video games it'll be like 1am, I'll yawn and suddenly think, oh I should get in a couple hours of writing. Whereas if I had already cleaned the house and emailed my family then I'll be in bed by 11.
It's a problem where the solution is to not have the problem in the first place, which doesn't help if you already have the problem.