What position were humans designed to sleep?

I've always wanted to stop sleeping with a pillow. It's obviously not the way us humans were designed to sleep. No "pillows" existed back during cavemen times. How did cavemen sleep back then?

I can only assume they would use their arm(s) as a pillow. If you lie down on your side (as humans are likely designed to do, otherwise the desire to sleep on our sides wouldn't exist), and you use your arm as a pillow, your arm is the perfect distance between your head and the ground to keep it in its correct position (which is the position it is when you're upright, just sideways.)

So after noticing this, I tried to figure out every possible sleeping position where I use my arms as a pillow in some way. Maybe bend my arm in a certain way, maybe this maybe that. Maybe use my hands? Nothing was comfortable though and I just used a pillow.

So I have to ask...how do I sleep without a pillow?