Certainly you can turn them off at night. Covering the trays in some way to raise the humidity will help germination. You can use saran wrap if it is far enough above the soil, otherwise the sprouts will hit it as soon as they appear.

Saran wrap also cuts down on your watering chores. I have stuck popsicle sticks in the soil to make a little tent out of saran wrap. That worked but was a pain, so eventually I bought some plastic domes made for the purpose.