"The boss sends at ten, so that's the culture"

Your manager sends emails at ten at night. Sometimes half past. That tells you what's expected, doesn't it. If the person above you works at that hour, the culture is that hour, and the fast reply is just reading the room.

The room does matter. Daantje Derks and colleagues followed a hundred employees over four working days and found that on days when people used their smartphones more for work in the evening, they reported more interference between work and home, and that the link was stronger where colleagues and supervisors were seen to expect availability after hours. The authors' advice was aimed at managers. Be clear about what you expect, and remember that the emails you send in the evening have recipients. So the norm is real, and a manager who sends at ten does shape it.

But sending isn't expecting. Your manager sends at ten because ten is when she clears her inbox, or because she has children and ten is the first quiet hour, or because she's on a train. None of those is a statement about when you should reply. If you asked her, she might well say she'd rather you didn't answer, because then she feels she has to answer back, and now two people are working at ten. The message at ten tells you when she writes. You've read it as an instruction about when you read.

There's a version of this belief that's harder to argue with. You've never asked, and asking feels like admitting you'd rather not be there at ten, which feels like the wrong thing to admit. So the culture is inferred from the timestamps and never checked. That's the most common way a workplace ends up with an expectation nobody holds. Everyone reads the timestamps and replies to them, and the timestamps multiply.

The fair objection is that in some places the boss does expect it, and says so. Chapter 22 takes that case. It's rarer than it feels, and the way to find out is one conversation, which chapter 18 sets out.

The practical implication is that a manager's sending time is information about the manager, not about you. What she expects of you is a fact you can get by asking, and until you've asked, you're guessing in the direction that costs you most.