I have thought long and hard on this subject and even explored it in conversation and here is what I came up with. I think in general our families do understand, but sometimes they get a bit jealous too or frustrated if you like that word better. I think they too get tired and feel they should be able to nap as much. What I don't understand though is why they don't just lay down and take a nap too. They always use the excuse they have to much to do.
But what irks me is that I get very angry and very frustrated when they make comments when I get tired and need a nap because usually I am pretty high energy. They act like I commit a federal offense or something. But I have learned to turn it back too!
Try looking them straight in the eye and remind them that you are disabled, you didn't ask to be that way, and that if you had more to give, you would. If they can't get that right, then shut the door and smile and get some sleep anyways.
Besides a soul can't do anything for anyone if a soul doesn't take care of itself. So if you need time to power your batteries, take it. There is no sin in it and if someone gets upset over it, then there is sin and it belongs to them for asking you for something you don't have to give. Let them own it too, lol, I sure know I have enough of my own sins that I don't need to own their's too. ahhhahahaha. Enjoy it!