It’s all that Cortisol (and then adrenaline) from the high intensity. That’s what keeps you awake.
I get the same, so do my mates that do anything hard/race on an evening. I just accept that I’m not going to get a great sleep if I do a hard workout/race. Which is annoying, as good sleep will give huge performance improvements!
When I used to TT, I’d be wired (plus is a great mood) for hours! I’m also usually too warm in bed too. But the next day, hanging.
If I’m planning a hard workout, I try and do it earlier in the evening if it’s possible, then chill out for a couple hours before bed. But can’t do that with a race.