We’ve all been in games (sometimes weekly) where we sit down and get immersed into a game so much that 4 hours seems to just fly by. For some however, the ability to remain seated is difficult. If the need to rise from ones chair is controllable to a degree, then allow players to stand and walk around during turn orders, only to return when they prefer, or when it is time for them to engage with their group. Allowing this ability and communicating it’s availability at the outset of your session could alleviate some of the triggers that cause the need to leave to begin with.
Additionally being out of one’s seat may allow for increased opportunity for active roleplay. Just ensure that you create touchstone points, where it is preferred that the player return to the table, even if standing.
Ensure that the environment is clear of tripping hazards, and if the need to remove oneself is due to a condition that requires access to other facilities, ensure that the path to that facility is clear.