"We're both adults, we can talk it through"

You're reasonable people. You've resolved hard things before. Two adults who love each other can sit down and talk something through at any hour, and the idea that you can't do it at midnight is patronising.

Clare Anderson and Charlotte Platten, in 2011, kept people awake for one night and then tested how they responded to negative stimuli. After one night without sleep, people were more impulsive with negative things. They failed more often to hold back a response, and when they responded wrongly, they did it faster. The researchers noted that sleep loss had already been shown to intensify the brain's response to negative things, and their study showed what that looks like in behaviour. Less holding back, quicker mistakes, specifically toward the negative.

William Killgore, in a 2010 review, went through what sleep loss does to thinking more broadly, and found that while some kinds of reasoning hold up fairly well, the parts that depend on the front of the brain, and the more flexible and creative parts of thinking, are the ones that degrade. An earlier paper by Seung-Schik Yoo, Matthew Walker and colleagues, in 2007, had a title that said it plainly, a disconnect between the prefrontal cortex and the amygdala without sleep, which in ordinary words is the brakes coming off the part of you that reacts. I've read only the title of that one and the notes say so.

Now, you haven't been awake all night at eleven. But you've been awake since half past six, you're in Golder and Macy's low hour, and the row has been running for an hour, and every one of those findings is about the direction things move as sleep is lost, not about a threshold you're safely under. The adults on the sofa at one are not the adults at the kitchen table at eight. They have the same values and the same love and less of the equipment that lets them use either. The flexible thinking that finds the third option is the first thing to go. The holding back that stops the sentence about their mother is the second.

An earlier book in this series is about anger and its rule is nothing said or decided in the first twenty minutes. This book is that rule at the scale of an hour. Not because you're not adults. Because adults at one in the morning are running on less, and the argument doesn't know that and doesn't care.