Try and enter sleep with as much calm as you can, so maybe deep breathing exercises. That's a beautiful way to calm your sympathetic fight or flight system is deep breathing. ... But if there's a way you can make your sympathetic system, nervous system calm down before you go to sleep, might free for you meditation, or deep breathing exercises might be for some, a warm bath, or a comforting book, nothing too exciting but also nothing too boring, perhaps, just something right in the middle which makes you feel happy and calm is what you should do.