Why do I wake up every night drenched in sweat?

I've been having problems with waking up in the middle of the night covered in sweat for no apparent reason. My boyfriend is the same way. I go to bed after him, but when I climb in, I have to take the covers off him because he is covered in sweat. We have the fan from the AC on all night as well as an oscillating fan and it's wintertime where we live, so why is this happening? I thought it was because we get hot in the middle of the night from the heat being turned on (by our apt complex) but I have tried sleeping in nothing just so I can hope that I don't wake up all covered but it hasn't been helping. Can anyone help pinpoint the reason and maybe a solution?