This sounds like sleep paralysis to me.
Most people experience it a couple of times in their lives, but for me I experience it a lot!
It's a really uncomfortable and eerie feeling and, even though it's harmless, I can get it non stop whilst going through a lot of stress.
I open my mouth to call on my mum, trying to scream at the top of my lungs but nothing comes out. At that point I feel as if something really sinister is in my room and I'm trapped in my head unable to move my body.
I don't know if it is more common in people with bipolar, but I do remember reading somewhere that it's only a couple of times in their life an average person experiences it.
But, for you and I, experiencing it a lot can be a pain :/
I try to lie on my side now at night, I get them less if I do that.
Hope this helps
Hypnagogic and hypnopompic hallucinations are symptoms commonly experienced during episodes of sleep paralysis. Some scientists have proposed this condition as an explanation for reports of alien abductions and ghostly encounters. A study by Susan Blackmore and Marcus Cox (the Blackmore-Cox study) of the University of the West of England supports the suggestion that reports of alien abductions are related to sleep paralysis rather than to temporal lobe lability. There are three main types of these hallucinations that can be linked to pathologic neurophysiology. These include the belief that there is an intruder in the room, the incubus, and vestibular motor sensations.