system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When laying in bed, what provokes the need to roll onto one specific side?
gpt
Well do you mean the first position you roll into after laying down or the constant change in sleeping positions?

 The latter is, biologically speaking, the need of your body to feel comfortable to prevent pressure ulcers (lat. Pl. Dekubiti). And to remain concious of your own body.
Since sleeping phases are in relation a long period of lying down, your body has to constantly remind itself where it begins and ends. 

Also to prevent certain nerve or bloodvessel from being surpressed too long (everyone knows the feeling after a body part fell asleep, because of missing proper bloodcirculation).

And our instinct kicks in from time to time to roll onto the other side for hearing in that directions if predators sneak up on us.

Do you need sources with that?

If you meant the first position you roll into instinctively after laying down, it usually comes with habits from your childhood and wether you're left or right handed, though that is a thesis which is not properly researched.

I hope this helped.

Grammar and spelling errors may be eaten.