My granddaughter has stayed with me 3-4 nights a week since she was born (and is with me 6-7 days a week besides). She is just over 3 years old. When she is sick with a cold or other bug, she wants to lay in my bed with me for a few minutes of comfort, but then she wants to go back to her bed. Sometimes when she is having trouble breathing, we will sit in the rocking chair and she will doze with her head on my chest, but then she awakens and wants to sleep in her own bed. Only when she was a newborn did she ever sleep a full night in my bed with me. Once she realized that she had a bed of her own, she wanted to sleep in it, even when I preferred that she sleep with me. She has never been one to fall asleep on the floor while playing, or while watching a movie--she has always let me know that she is sleepy and ready for "nappies."
She also wakes up if someone puts a key in the door lock, or touches the end of her bed--none of that "dead to the world" deep sleeping for her. She wakes up if a fly changes direction in mid-air! I have never seen anything like it--her daddy slept through diaper changes and even through middle of the night (necessity) baths!
And on that train of thought--I sleep like the dead. I have slept through hurricanes, earthquakes, and even being carried by a fireman out of my burning bed and being set into the snow as an adult.
When I still had my granddaughter sleeping in my room with me, I woke up whenever she turned over or stretched her legs. Now that she is in her own room, I can hear her if she clears her throat or starts whimpering in her sleep.