Sometimes when they learn a new skill, such as sitting up by themselves or standing, they can do it in their sleep for some time, and not work out how to get back down.
Yep, dd would stand up on our bed and walk. I'd wake up because I'd feel her move but she was actually petrified of walking without assistance during the day. She'd practice in her sleep on the bed. Scared the hell out of me one night when I felt her at my feet and she was about to step off the bed. Maybe a toddler bed maybe the way to go. Even if you move her cot next to your bed so she can hear you breathe?

HTH