What is causing this ravenous 3:00 AM hunger?

It started a few weeks ago and it can't be ignored, I have to get up and eat. The only thing that assuages it is protein and lots of it(normally my cravings are for sweets but that has all but disappeared). I'm not gaining weight because I'm not hungry during the day. I get hungry again around bedtime. The lack of sleep is really affecting my life. What's causing this?