system
A chat on subreddit /r/NoStupidQuestions
human
Because you are relaxed on the couch and falling asleep has no commitment.  You don't expect to fall asleep and if you do you will wake up still in the evening and do more.

But falling asleep in bed is to end your night and you will wake up in the morning - which for most people means work or school.  

So falling asleep in bed is an ending that leads directly to work or school.  There is stress in that rather than relaxation.
gpt
This feels intuitively true. I don't sleep nearly as well when I have to be up early. It seems like a common enough phenomenon that I think it merits its own scientific name.