I don't know if you saw the patch notes, but sleeping was specifically removed. I did see the computer still mentions sleeping in the tutorial, but that's probably an oversight. I am going to check if staying in bed boosts stamina regeneration, but it seems to me it's only placebo levels. Hope that clears things up!
I got the results from my test. At level 15 I could regenerate full stamina in 1 minute 40 seconds by standing around. By "sleeping" in bed (just moving into it) I could fill it in 50 seconds. So technically sleeping is still a usable feature.
It still only took 18 seconds for me to use all my Stamina on some Air Cyclers though lol. Definitely needs to be balanced or have a patch testing no Stamina.