Sitting on something (chair, sofa) then pressing the wake up button (Esc) would not wake you up until you stop sitting (in my case standing up chairs/ATV is binded to Q, so if I'm sit-sleeping I need to press Q then Esc to wake up)
Separately, player model just stands there when you're sleeping (1), disappears if you sit (2). No idea what (3) is, but that's how Kel looked like after sit-sleeping (crouching? shrunken?)