This is pure speculation on my part, but I have an old back injury in my left lower back. I have no idea what it is and neither do a ton of doctors that looked at it.
I know that stretching my lower back (toe reach, toe reach with one foot over the other, especially when stretching the affected side) helps immensely.
I noticed that when I wake up with my back stretched out (i.e. don't feel a stretch on trying it) I feel very well rested. On other days, I feel completely tired. This does NOT seem to be related to amount of sleep, but to the position my back was in when I slept (avoiding lordosis).
I know this is a vague, but is there any evidence to support my theory that my back alignment is affecting how "tired" I feel?