Exercise - lots of it for DS - or else the whole family would be up all night. So, we enrolled DS in a year round swimming program (indoor pool) and martial arts - and he has spent enough time to advance to levels where these activities get intense and physically demanding. This really helps in tiring him out. And then, we have a no screen time at home rule. That helps too. And the third thing that helps is a routine - bath, reading in bed and lights out at a fixed time.
My friends swear by chamomile tea and lavender tea for kids before bed - but I have not tried them so far.
