I'm curious about the waking-up-at-3am phenomenon too. I've suffered from it and read about people experiencing the same thing in different sections of Mayo Clinic Connect.

I've tried researching it and talking to doctors and specialists about it, but I haven't found any answers. I'm a night owl, so I was going to sleep at about 1:00 a.m., while others were heading to bed at 11:00. My solution, which isn't really a solution, has been to go to sleep later and later.

When I was waking up around 3 or 4, my heart would be racing, my BP would shoot up, and I would have "night terrors"—thinking there were snakes on the floor and/or sinister shadows lurking behind my door. I'd have to put all the lights on and sit up until I felt safe again. It was awful!

I started going to bed later and later. The sweet spot has been waiting until dawn started to break. That way if I woke up after a few hours, I could see that there was no one in my room nor snakes on the floor and go back to sleep. I'm retired now, so I can keep these strange hours, but obviously most people can't. I also take a lorazepam (1.5mg) just before going to bed, which seems to help. I'm still having sleep problems, but the rapid heartbeat, the high BP, and the waking nightmares have subsided, thank god.

I do have severe obstructive sleep apnea and some central apnea, but I've used a bi-pap religiously for 15 years and added oxygen a few years ago due to a diagnosis of hypoxia. I've had multiple sleep studies, but because the only hours I can get one are 9pm-5pm, they don't reflect my regular sleep schedule. Because of this, I don't cycle through my normal stages of sleep during the studies, and the results aren't necessarily what they would be on a normal night at home.