Yes, it's just a harness that attaches to the lap belt and goes over the seat behind the child. I can't say for sure if a child could sleep comfortably in it since my DD has never slept on a plane or in a car in her life

I can say she was much more comfortable in the CARES than her car seat for almost 3 hours. She had more room to move around in the seat, play with her toys, look out the window. Her car seat kept her held back and the sides are high on hers so she couldn't see out or move too well...which is actually good for a younger baby! It also was great since she couldn't reach the seat in front and didn't kick it the whole flight.
I wouldn't recommend it for a baby, in fact I don't think you can use it until they are over 2 maybe? My DD is almost 3 now (but a peanut in the 5th percentile of height/weight).