I (lifelong side sleeper) started using a (dedicated) knee pillow about a decade ago and I am completely torn about them. On the one hand: amazing. Better spinal alignment, zero sensitivity or soreness where the knees meet, better sleep, easily the best $10 I ever spent. On the other hand: I can no longer tolerate the weight of my knees on each other for even a minute. Can’t sleep or even just side-lay for more than thirty seconds without a pillow or a folded blanket or something between my knees without it aching like hell and being too uncomfortable to bear. From speaking to others that adopted knee pillows, it seems to be the same: once you use them, you can never go back.
So now I marvel at how I used to be able to sleep without them and ponder over why we either have no fatty pads (or muscle, even!) on the inside sides of our knees or have reduced sensitivity in that region, since we’re clearly meant to be side sleepers (it’s the healthiest position, assuming you can find a rock or log to place under your head, or even just your hands folded together).
The book I have on how to sleep properly says that on your side you hook your big toe around the Achilles' tendon of the other foot, this offsets the knees.
But it also said to sleep on the floor, and no thanks.
So now I marvel at how I used to be able to sleep without them and ponder over why we either have no fatty pads (or muscle, even!) on the inside sides of our knees or have reduced sensitivity in that region, since we’re clearly meant to be side sleepers (it’s the healthiest position, assuming you can find a rock or log to place under your head, or even just your hands folded together).