Does only 1 survivor go to sleep at a time? I've seen some vids of it and I've never seen survivors all go to sleep at once.
Each survivor has a separate timer. There's nothing preventing them from all going to sleep at once, but their timers can easily fall out of sync if they fail a skill check or after they wake up.
Assuming you start a match and nobody fails a skill check or gets hit, everyone will fall asleep exactly 60 seconds into the match.