What time do you give the Intuniv? Is he still hyper when it's in his system and then crashes (rebound effect) when it wears off? Everyone's heartbeat slows down when they're sleeping. I would definitely talk to the psychiatrist as soon as you can. I would absolutely NOT increase the medication again until this deep, nose-diving crash is figured out. It could be the wrong medication, too high a dose, long-acting when a short-acting might work better ...... There are so many possibilities. I know it's scary reading those darn side-effect sheets sometimes but you need to somehow calm down and try to stay level-headed. difficult child 1 is on the short-acting version of Intuniv as well as Strattera. The combination works great for us.
Get some sleep. Call your psychiatrist first thing Monday morning, unless they have a 24-hour call number.