Case 1: When I sleep slightly less than would be ideal, for example less than 7 hours per night, I wake up reasonably functional and get through the day without being excessively tired, if not being totally alert either.
Case 2: When I sleep slighly less than would be ideal, for example less than 7 hours per night and wake up, let the dog out to the backyard, wait a few minutes, let the dog back in, and decide to go back to sleep because it’s weekend. Now falling asleep takes a while but eventually I succeed. Then after an hour or two at max, I wake up again, but this time around I’m extremely drowsy with muscles aching and it takes serious effort to get up and get the engine running at normal speed. Even though the total amount of sleep (about 8-9 hours) sounds pretty ideal on paper.
Why is that?
It’s probably related to sleep cycles, and maybe the issue is waking up at the “normal” time coincidences with the “right” cycle time to wake up, and when sleeping in you wake up because you’re topped off, or have to drink or pee or whatever, which may or may not be at a good cycle time.
This sounds plausible.