It's 10:40 on a Tuesday night, you finally have a clear head, and you remember the thing you meant to tell a coworker. So you open Slack and type it out, and your thumb hovers over send, and a small voice asks: is this rude? You're not asking them to do anything right now. You'd be horrified if they replied at 10:40. You just want it off your plate before you forget. But you can feel that hitting send at this hour says something you don't mean.
It does say something. That's the whole issue with messaging after hours. The words might be "whenever you get a chance," but the timing speaks over them, and what the timing says is "I'm in work mode right now, on a Tuesday night, and here's a little piece of it landing in your evening." Your coworker sees a work notification at 10:40pm. Most people, even the ones who swear their notifications are off, feel a faint tug to look, then a fainter one to deal with it. You didn't want to create that. You did anyway, just by sending when you sent.
And it compounds. Message people at night often enough and you quietly teach them that night is fair game, that this is a team where you're sort of always reachable. Nobody decided that on purpose. It just accretes, one 10:40 message at a time, until the weekend has a low hum of Slack running under it and everyone feels a little less off when they're off. You can be the person who adds to that hum or the one who protects the quiet. The second one is who people are grateful for, even if they never say so.
This matters more the more senior you are. A peer pinging a peer at night is mild. A manager pinging someone who reports to them is not, because "no rush" from your boss never fully reads as no rush. Some part of the brain always hears expectation in it. So if people report to you, assume your after hours messages land with extra weight, and act accordingly.
The fix is mostly mechanical, and it's the best habit you can build: schedule the message. Slack has a "Schedule for later" option hiding behind the little arrow next to the send button. Write the thing at 10:40 while it's fresh, schedule it to arrive at 8 or 9 the next working morning, and you get the best of all of it. Your plate is clear, and their evening stays theirs. The message is just waiting politely when they sit down with coffee, which is when they'd have handled it anyway. Half the people reading this don't know that option exists, and it solves the whole problem in two clicks.
Sometimes you do want to send it now. You're heads down, you want it out of your head, fine. Then say the quiet part so they don't have to guess: "No need to look at this till Monday, just getting it down while I remember." One line, and the pressure drains out of the message. You've told them, in writing, that the clock isn't running.
But keep that line light. The instinct is to apologize: "I'm so sorry to bother you this late, I feel awful, ignore me!" Don't. Like every apology piled on too thick, it draws a big circle around the very intrusion you're trying to wave off, and now they're thinking about the lateness more, not less. "No rush, Monday's fine" does everything the grovel was reaching for, without the flinch.
One honest caveat. If the thing really is on fire, the site is down, a client is melting, then "no rush" is a lie and a Slack message is the wrong tool anyway. Call them. Say it's urgent. Reserve real urgency for real emergencies, and the rest of the time let the timing match the stakes, which are almost always "this can wait until morning."
None of this is about being precious. It's a couple of small habits that keep you from being the colleague who can't let Saturday be Saturday. Write it whenever you want. Just send it, or schedule it, like someone who knows the difference between your own work hours and somebody else's life.