"I'll just clear a few emails"

The evening session has its own justification, separate from the long day at the office. You're not really working. You're on the sofa or at the kitchen table, half watching something, clearing the inbox so that tomorrow starts clean. It feels light and it feels useful, and it feels as though you're getting ahead.

Look at what happens in it. You answer fourteen emails. Some of your replies reach people who are also clearing their inboxes, and they answer, and you answer back. By twenty to eleven you've handled thirty messages and your inbox has about as many in it as when you sat down. An inbox refills as you empty it, and every reply you send invites another. Evening email mostly produces more email. Almost none of it needed dealing with before morning, as you'd find if you checked tomorrow which of tonight's messages had been urgent.

Then there's the condition you do it in. By chapter 4's arithmetic you've been awake for fifteen or sixteen hours. It's the worst time of day to be making judgements or writing anything that has a tone. The message you dash off at ten at night is the one most likely to be misread or regretted, or to need a second message to put it right.

The larger cost is to the evening itself. Liuba Belkin, William Becker and Samantha Conroy surveyed 570 working adults about email outside working hours. They found that when people felt expected to monitor work email in their own time, they were less able to switch off from work, more emotionally exhausted, and less satisfied with the balance of their lives. The striking detail is that the amount of time people spent on email after hours wasn't the main driver. The expectation of having to check did the harm. A person who might have to deal with work at any moment never fully stands down, even on the evenings when no email comes.

In your case, nobody imposed the expectation. Helen has never asked you to answer email at ten at night. You built the expectation yourself, one prompt reply at a time, and the people who email you have adjusted to it. Because you made it, you can also unmake it. People's expectations follow your behaviour quite quickly. If your replies start arriving at nine in the morning, then within a few weeks that's when people will expect them.

The evening session also wrecks the hour before bed. You close the laptop at twenty to eleven with your head full of the rota and an irritating message from the finance team, and then you try to sleep. If you sleep badly, that's an obvious place to look.

What the session really provides is relief from the thought of an unattended inbox. You can get the same relief far more cheaply. Ten minutes at the end of the working day, triaging what's there and noting the two or three messages that matter for tomorrow, will do it. Once the working day has ended, what's in the inbox is a matter for the next morning.