What can we do to keep cat from going off at night?

The last 3 months our cat will just sound off at random times of the night. We've tried everything that is humanely possible and are at out wits end. I've tried to keep him busy during the day by playing and giving him things to play with. At night when we go to bed sometimes he sounds off, but most of the time it can be anywhere from midnight to 4 or 5 in the morning. Usually around 3 is one of the times he sounds off. We've also sprayed him with water, pet him, shook him a little to let him know we're not happy, let him sleep beside my side which is getting increasingly difficult since I'm pregnant and getting bigger, and I've even taken him to the kitchen sink and threatened him with water that way. I've given treats thrown a pillow and an empty water bottle at him. Even after some things he still will sound off. What can we do? Help! I'm/we're frustrated and disgusted and loosing even more sleep than we should.