Energy Used While Sleeping

so how much of that is bodyfat?

will the liver convert bf to glucose while sleeping?

A high percentage of it will be from bodyfat. You can pretty much think of the fuel source used for energy on a sliding scale depending on the level or intensity of activity, but always using a combination of sources.

Fatty acids won't be converted to glucose, the body uses them directly for fuel. There will always be glucose in the blood stream for use by the brain, etc. But stored triglycerides will be hydrolyzed from the adipocyte and the free fatty acids will be released into circulation and carried to the tissues that can use them as an energy source (skeletal muscle, cardiac muscle, liver).

Yeah - what he said. 

Liver glycogen will be used as well, but virtually no skeletal muscle glycogen.  Fatty acids and liver glycogen. 

so if structured properly with little carbs at night it's a good time frame to lose some body fat?

Absolutely.  Though body temp and met rate is down - and obviously activity - you're still burning 30-50 calories an hour and a lot of it is from body fat.....especially if you're not going to bed with a gut full of carbs and you are in a calorie deficit (as you would be during all of contest prep).  You're not eating for 8-10 hours - that's a lot of energy being used from fat.

Would it be best to take a slow digesting carb like a little bit of sweet potato or maybe a few glasses of milk?

A "few" glasses of milk and carbs before bed?  If gaining weight is your goal; that's a perfect plan.  Toss some ice cream in the blender with that milk and a little Hershey's syrup and you're an anabolic machine!

Hmmm...maybe I need to rethink my pre-sleep routine. Could be why I wake up in a pool of my own sweat. I'm an INFERNO!!! ;)