Remember you are healing... you are giving the digestive system a chance to rest, and heal. It no longer has to spend all of it's energy in "daily combat" with digesting foods, bad diet, and continually playing catch-up. Now it can actually "get ahead of the game", and start healing itself and improving things.
I have noticed the same thing, I've been doing the Bowel Cleanse and my sleep has gone up... I guess my body needs it to help build itself up and/or heal things. I don't worry about it, I just "go with it" and let it lead me to wherever it wants to go!