"I feel great on a Sunday"

You do, and the feeling is the most persuasive thing the habit has, so it needs a proper look rather than a dismissal.

By Sunday lunchtime you've had two long nights. Some of the short week's shortfall has been paid, as chapter 3 said, about an hour of it in the Depner study, and you've had a slow morning with no alarm, and it's the weekend. Of course you feel good. The question isn't whether the feeling is real. It's what it's evidence of.

It's evidence that you were short of sleep, and that two long nights helped. It's not evidence that the two long nights were the best way to get them, and it's not evidence that the feeling will last. Look at the shape of your week as a graph. Friday evening low, Sunday afternoon high, Sunday night bad, Monday low, Tuesday middling, Wednesday fine, then a slow decline to Friday. The Sunday peak is real and it's paid for by the Monday trough, and the two are about the same size, and you'd have neither with a flat schedule. What you'd have instead is a week that sits somewhere between them, a little lower than Sunday's peak and a lot higher than Monday's trough.

Here's the piece that's hard to feel from the inside. The Sunday-afternoon high isn't only recovery. Part of it is that at three on a Sunday your shifted clock is in its natural daytime, wide awake, well lit, because your night now runs later. You feel sharp on Sunday afternoon for the same reason you'll feel dreadful on Monday morning. The clock has moved, and on Sunday afternoon that's a tailwind. On Monday morning it's a wall.

I'd add an observation, and it's mine rather than a finding. People who hold a steady wake time through the weekend don't report a Sunday peak, and they don't miss it, because there's nothing to peak from. Their Saturday morning feels like their Wednesday morning, which for most of them is fine, and the weekend feels like two days off rather than two days of recovery. If that sounds flat, ask yourself what you'd trade for a Monday that felt like a Wednesday. I think most people, put the choice that way, would trade the Sunday high without much regret.