Yes it's OK to sleep outside at night.
Sleeping outside on a warm or cool night is perfectly fine and you'll likely get much better sleep due to the fresh air.
Sleeping outside is pretty safe as long as you're not sleeping outside in a dangerous city or a dangerous location where gangs and robberies happen.
If you're sleeping out in your backyard you're also likely safe there as well.
I have fell asleep in my hammock in my backyard one night and I woke up refreshed the next morning.
It was not my intention of sleeping outside but I fell asleep and didn't realized I stayed out all night until I woke up and realized I was still in my hammock.
Sleeping outside in a state park in a tent or sleeping outside on a tarp in a sleeping bag etc in the woods is also pretty safe as well.
Also camping in a tent is pretty safe.
Anything can happen but if you're out camping in the wilderness in a tent then it's unlikely you're gonna get robbed, murdered attacked etc.
Even getting attacked by animals in your tent when sleeping is very unlikely.
Also if you're tent camping in a state park then they usually have rangers that roam around and watch things.
So tent camping in a state park is even safer than tent camping just anywhere.
I've been tent camping for years and never have been harmed by humans or by animals.
If you can you might want to bring a gun just in case and you may need it for snakes.
But just be aware that some state parks may not allow guns.