system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come we don't have to get up while we sleep to avoid blood clots, but we do when we are awake?

like when you are on a plane or sitting at a desk for work, it's recommend that you get up and walk around everyone in awhile to avoid blood clots. Why not when we are sleeping?
gpt
Because you sleep horizontal - your blood is less prone to pool in your lower extremities since it doesn't have to work against gravity. The reason that idling at a desk or in any kind of sitting position can be risky is because blood flow in the lower extermities is assisted by the contraction of the muscles, which give a kind of 'pumping' action to your veins. Without activity, though, the blood just pools in your lower legs, and its that stagnation that risks blood clotting.

TL;DR - never sit when you can lay down.