"Sleep in" versus "Sleep out"

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

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.