"Someone else will answer first"

If you don't reply tonight, somebody else on the thread will, and they'll get the credit for being on it. Over time the fast repliers get noticed and the slow ones don't, and you can't afford to be a slow one.

I'll concede something here that the other chapters don't need to. Some workplaces do reward speed. Giurge and Bohns open their paper with the observation that workplaces increasingly use response speed as a proxy for hard work, and a proxy is a real thing, even when it measures the wrong thing. If your manager keeps a mental list of who answered the Sunday email, then answering the Sunday email has a value in that workplace, and pretending otherwise would be dishonest.

Now look at what "credit" comes to over a year. The person who answered at ten on a Sunday gets a flicker of notice. The person who delivered the deck on Thursday and it was good gets the project. Speed is noticed in the moment and forgotten by Wednesday. Work is noticed at review time. If you've got a fixed amount of attention to spend on being noticed, spending it on the reply is spending it on the thing with the shortest shelf life.

Then look at the race itself. If someone else answers first, what have they won. They've told the thread they're available at ten. They've taught the sender to send at ten. They've signed up for the next one. You can let them have that. The thing you've lost, in the version where they answer and you don't, is a reply nobody will remember. The thing you've gained is an evening, and a sender who has learned that you answer in the morning, which is chapter 13.

There's a harder version of this worry, and it's about early career. If you're new, or junior, or in a job where the next contract depends on being seen, the pressure is real and the cost of a reputation for slowness is real. My answer is still the same, but I'd hold it more lightly. Be reliably fast within a window that's generous, say seven in the morning to seven at night, and be reliably absent outside it. That's a person who's on it, not a person who's on it at eleven.

The practical question is what you'd want to be known for in eighteen months. Nobody is known for their reply times. They're known for their work, and the work is done in the day, which is where the reply belongs too.