Why Do I Get Leg Cramps In The Early Morning At Dawn?

I get leg cramps in the early morning hours after 3am. I have enough trouble sleeping. So to be woken up its hard to go back to sleep. I also have sleep apnea. I take a 40 mg lasix, but i drink 3 32oz mugs of water. Am i missing a mineral im not taking. i do take a multi vitamin.