A few weeks after getting a head injury I found myself lying on a sofa, putting my laptop to one wide and staring at the wall for the next few hours, my mind largely blank. In the that happened several times that year but hasn't happened for a good while now, I usually know when I need a rest and my subconscious doesn't need to make me do it.
According to my favourite head injury book Brainlash (https://www.amazon.co.uk/Brainlash-Maximize-Recovery-Brain-Injury/dp/1932603409) it's not uncommon and if it happens you should just let it as needed.
This week I had surgery on my eye to get rid of the double vision I gained along with the head injury. It was a general anesthetic and these things have a habit of taking away your physical strength. In the week since I've found myself stopping and staring a few times. My body knows when it needs to recover. Have some good music and a comfy bed ready seems to be a good idea.