The phrase "supposed to sleep" is correct and usable in written English.
You could use it to talk about someone's expectation or obligation to sleep. For example, "He was supposed to sleep, but he stayed up all night instead."
Our babies are supposed to sleep through the night, but both co-sleeping and the cry-it-out method are risky.
But then what side were you supposed to sleep on?
Where, she wants to know, is she supposed to sleep?
You're not supposed to sleep at a professional basketball game.
Was I supposed to sleep only every other night?
It was the first time I realized you're actually supposed to sleep.
So I've been popping generic Ambien (they gave me ten! So I'm supposed to possibly sleep one night of the month?).
Felber, who writes in machine-gun bursts of drollery ("When you can't sleep but you're not supposed to be sleeping, does it count as insomnia or not?"), wrings each character for a few laughs, then hustles on to the next.
We're supposed to be sleeping late to be rested for tonight's Soyuz flight home, but I'm finding it hard to sleep in.
It's called the Vividus, and it comes with two metal plates affixed to the mattress, in your choice of nickel or brass, inscribed with your name and your bedmate's name – so that, if you stumble home drunk, you can look to see which side of the bed you're supposed to be sleeping, like a place setting at a dinner party, and who you're supposed to be sleeping with.
Another student, perhaps assuming that Siyonbola was an intruder or disturbed at the sight of a black person sleeping in plain view (or both), turned on the lights and told her, "You're not supposed to be sleeping here.
