For me I find it is usually cuz my feet have gotten hot. I slepp 10pm to 4 am great, then wake at 4am feet burning and swollen. I guess I believe I pull my feet in under the blankets as I sleep with a fan on them so they get cold and I pull them in and I wake with them hot.

But, it's a crap shoot. I was also told its and adrenal / cortisol issue that I wake at the same time every night.