Falling out of bed

For the past few weeks my daughter has been falling out of bed when she's asleep. She will sit up in bed asleep and drift off to the side and fall out of bed. Has anyone had anything similar happen before? We have a bed rail if we need to use it but she's never needed it before. She only has a small child single bed so it's not massively high off the floor but I'm worried she keeps hurting herself when she falls off. Sometimes she does fall straight back on to the pillow but has hit the floor a few times. I'm not sure if the bed rail will do anything as I'm worried she will just hit herself on that instead

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