You have been on manual labour mode all day. Perhaps you've been digging the garden, building a brick wall, doing some serious DIY or renovations. Whatever it is, you've been working bloody hard and at the end of they day you have five minutes to catch your breath. You sit down into a nice comfy armchair and close your eyes for a few seconds.
9 times out of 10 you will be fast asleep before you know it, but however you spend it this chill out time is some of the best there is. A rest is better when well-deserved.