In the novel, they've had a pretty good night. Several prostitutes were smuggled onto the ward along with a considerable amount of alcohol. There's some discussion of maybe escaping to "Mexico or Canada" but McMurphy and Turkle decide to have a bit of a snooze before they put their plan into motion despite the window being wide open.
Harding was supposedly going to wake them up at 6am (note that it's already gone 5am when the discussion was happening) so they could leave before the orderlies arrive at 6.30am but what actually happens is that they all end up sleeping late.
He squinted up at the dim clock. “It’s nearly five. I need me a little
shut-eye before my big getaway. The day shift doesn’t come on for
another two hours yet; let’s leave Billy and Candy down there a while
longer. I’ll cut out about six. Sandy, honey, maybe an hour in the
dorm would sober us up. What do you say? We got a long drive tomorrow,
whether it’s Canada or Mexico or wherever.” Turkle and Harding and I
stood up too. Everybody was still weaving pretty much, still pretty
drunk, but a mellow, sad feeling, had drifted over the drunk. Turkle
said he’d boot McMurphy and the girl out of bed in an hour.
I shook his hand, and we all started for the dorm. McMurphy told
Turkle to tear up some sheets and pick out some of his favorite knots
to be tied with. Turkle said he would. I got into my bed in the
graying light of the dorm and heard McMurphy and the girl get into his
bed. I was feeling numb and warm. I heard Mr. Turkle open the door to
the linen room out in the hall, heave a long, loud, belching sigh as
he pulled the door closed behind him. My eyes got used to the dark,
and I could see McMurphy and the girl snuggled into each other’s 
shoulders, getting comfortable, more like two tired little kids than a
grown man and a grown woman in bed together to make love.
And that’s the way the black boys found them when they came to turn on
the dorm lights at six-thirty.