when i lay on my back i get itchy

[Sentry]

Usually having dry skin will make you itch in random areas of your skin.

[Ad Bot]

[Caribou]

If you have dry skin, that can happen. It also happens when you sun-bathe.

[ResonKinetic]

Every.
Single.
Thread.

Hey, I'm no doctor, but I can share what I know and have experienced.

When my albeit limited expertise runs out, I suggest medical help from a medical professional.

[GhostStar]

i don't know about itching, but sometimes when you are falling asleep your brain makes muscles in your body twitch and jerk, maybe kinda the same thing with itching?

[sir dooble]

You don't have any insects in your room at all?
Little gnatty flies perhaps? If they land on you they can tickle and get itchy.

They can also bite too, which makes you itch.

It's not always very likely, but your bed may have bedbugs. But you may notice these from visible bites, possibly blood smearing on sheets, seeing the bugs or seeing their shell casings.

Also, do you have any pets?
You might want to check that they don't have fleas. They can be a nuisance and bite too.
They're generally easy to get rid of with flea medicine and flea collars though.

[TristanLuigi]

I thought this was super common.
This happens to me if I take my shirt off when I sleep but I usually wear a shirt when I sleep.