#energy levels

Some of the most satisfying sleep I get is when I am in the middle of working, find myself needing to lie down for a micro-nap, just 2-3 minutes, and then accidentally sleep for much longer. Usually this is in an uncomfortable setting, such as on a hard floor, and in an unsustainable position, such as one that puts your arm and/or leg to sleep. COnsidering all this, why is it that this sleep feels more refreshing than a typical night's sleep in a bed? Maybe the answer is just that those improvised naps are the times when I need sleep the most.

Unrelated question: why does my perception of physical wellbeing vary so wildly from day to day, while my daily habits regarding sleep, diet, and exercise barely change?