Lymph is the non-blood liquid that surrounds and is sometimes in some of your tissues. It’s related to the circulatory system through osmosis and similar processes but it moves around a lot better if you move your body. It’s sampled by your immune system, particularly in lymph nodes, which also act as a reservoir for various WBCs.

If you have a very sedentary lifestyle then your lymph doesn’t move around very quickly and so the processes it’s involved in are less efficient. This can mean a less effective immune system, fewer nutrients and more waste products near various tissues, and self-sustaining processes like localized inflammation cycles that stick around longer than might be ideal. This is one of the reasons that doctors usually recommend movement after injury, not just being sedentary and resting (the usual other reason being preventing scar tissue). Many of your body’s processes work better if lymph is circulating due to moving your body.

I mentioned it in this case because brain fog can be caused by many things and some of them are related to the processes the lpjatic system is responsible for.