Similar problem here (the adrenaline, not the brain damage). Sometimes I'm forced to the late practice because of work. I end up tossing and turning until 3am, especially if randori was as tough as I like it. I've tried many of the tips above and more, but nothing works reliably. I'm guessing the body just needs time to deal with all the adrenaline, which is probably why sleep doctors always advise against strenuous exercise within X hours of bedtime.
I used to have major issues with getting to sleep, for me I need about a 3-4 hour gap between excercise and sleep. What I find really helps me is eating bananas, milk, cheese as they help me go to sleep, I think they contain some chemical which helps with sleep, turkey does as well but I don't often have turkey lying round. I don't think getting hit is keeping you up if thats what your suggesting I just think that the physical exercise is winding you up.
This may take a while if you're at a new school, but this is what we do at my gym.
Class will run from 6:30 until 8:30 or 9:00, and all the sparring/free rolling takes place during the last half-hour. Once finished, my instructor will informally "close" the class, but let us stick around. During this time, we'll all hang around the mats, talk, run through techniques slowly, etc... We'll do this upwards of a half-hour on occasion.
This may sound surprisingly silly, but the simple act of transitioning your training partners from "Practice Opponent" to "Friend" does a lot to shift your perspective out of the adrenaline crazed rush that keeps you up at night.
Of course, you need to befriend your training partners first and work to cultivate that type of atmosphere in your school.