“Sleep in” versus “Sleep out”

Question

Over the years, I have often debated whether the phrase is "In the morning, I'm going to sleep in." or "In the morning, I'm going to sleep out." My best guess is that it is a regional difference of convention.

Is it? Are there any real, historical or otherwise, reasons to prefer one over the other?

(It would be particularly nice, arm-chair linguistically, if it were "I will sleep out." but "I slept in.", though I quite doubt that fits anyone's usage.)

Answer 1

The normal American English would be sleep-in if you plan on doing it on purpose or over-sleep if you do it on accident. Confusingly, sleep-over is different and refers to sleeping at somebody elses house. In the usage you describe, sleep-out is just a variant of sleep-in. The references I found were either British or ESL usage that looks like a second-language mistake. It doesn't look like it's a very common variant, you should stick to sleep-in. Here's the ngram

Answer 2

I would say, no, it wasn't regional usage, and they aren't preferred over one or the other. They're just used in different places. "sleep in" has a different meaning from "sleep out".

"Sleep-out":

–noun
2.
a person who lives elsewhere than at the place of employment.
3.
an act or instance of sleeping outdoors.

"Sleep-in" :

(intransitive) (idiomatic) To sleep late; to go on sleeping past one's customary or planned hour.

So, if you were going to "sleep late in the morning", I would use "sleep in", but if you were going to 'have a nap in the garden', I would use "sleep out".

That's my two coins.