system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can I fall asleep easily on one side but not the other?
gpt
Over time you have perhaps had deeper, less interrupted sleep on your preferred side. This could be down to the location of your window and the direction the sun rises in your current or previous abode. You could also have been woken up more by chance factors (partner/outside noises) while sleeping on your other side. Your body then subconsciously prefers the side which generally offers more rest.

This mechanism will have come from cave dwelling (light coming from one direction)

This is a complete guess.