I used to do it all the time. That is, I used to pray the Rosary when I climbed into bed, and I'd fall asleep before I had completed my prayers.
It has been a long time since I did that. But, it happened to me last night.
The first couple of times I woke up with the Rosary laying beside me on the bed, I felt like I had let God down. Like I couldn't "tarry with Him for even one hour" as it says in Holy Scripture.
But then I realized that it was okay. I couldn't let that bother me or I might limit how often I pick up my Rosary beads. I might let fear-of-not-finishing derail a whole lot of prayer time - and all the good that comes with this powerful prayer.
So, now I think of it in a completely different way. I pray with Our Lady, and if I fall asleep before I'm done, I know she completes the prayers while I sleep peacefully in her arms.
Do you have a Rosary beside your bed? Try picking it up, even on nights when you think you are too tired to finish five decades. When you wake up and find the Rosary right beside you, I'm telling you, it will make you smile - like the memory of falling asleep in your mother's lap or those times daddy carried you into the house in his own arms - and you can't remember how you made it back to your own bed.
As St. Therese said, "It's all grace." Even half-finished Rosaries.