It is rude to Slack after hours. Not always unforgivable, not always fireable, but rude in the way that compounds. The unread badge at 21:40 in Berlin is not “async.” It is a small flare on a night desk. The same flare in Tokyo can feel like hierarchy. In New York it often masquerades as hustle. In London it arrives with an apology the next morning that nobody should have had to write.
The test is not whether *you* are still working. The test is whether they are in a pingable window, on a declared exception, or asleep. Asleep.today exists to answer that without a lecture. This article is the lecture anyway.
After hours is local, not organizational
A company “HQ timezone” does not dissolve local nights. If HQ is New York and the engineer is in Berlin, 17:30 ET is 23:30 CET in winter. You did not send a late message to the company. You sent a late message to a person in Schöneberg. Feierabend is the name of the boundary. Honor it.
Japan’s leftover lights in Marunouchi are overtime, not an open office. A mention after 21:00 JST is remembered. London’s sorry-culture will still tag the timestamp. New York finance may answer at 22:00; a Brooklyn product designer will not, and should not have to.
If you do not know the local verdict, open the city page. Tokyo and Berlin will disagree with your laptop clock. Believe them.
The three exceptions
1. On-call. Named humans, named hours, named severity. “Sev-1, no action if you are not on the rota” is a complete first line. 2. They asked. “Ping me when the build lands, even if it’s late” is consent for that thread, not a lifetime pass. 3. Schedule send. You wrote it at 22:00 your time. They receive it at 09:28 theirs. You got the thought out of your head without stealing their night.
There is no fourth exception for “I just work later than you.” That is a you problem. There is no fifth exception for “we’re a global team.” Global is the reason to be *more* careful, not less.
How after-hours messages go wrong
They stack. You send at 21:12, follow up at 21:40, @here at 22:05. The recipient wakes to a crisis aesthetic around a non-crisis. They also go vague: “Got a sec?” at 23:00 is worse than a complete paragraph at 09:30, because it demands an immediate context switch with no payload.
They ignore holidays. US Thanksgiving is not a UK holiday; Boxing Day is not a US federal day; Golden Week is not on the Google US calendar. After-hours plus a holiday is how you look like you do not know the person exists outside Jira.
Compare a transatlantic pair: the New York–London overlap is fat enough that you rarely need nights. If you are slacking London at 20:00 ET, you missed the window on purpose.
A policy you can paste into the handbook
Working hours are local. Quiet hours follow the city clock, not HQ. Default send is schedule-send to 09:30 local of the recipient. Mentions after 18:00 local require incident language or prior consent. Recurring meetings that sit outside 09:00–18:00 local of any required attendee must rotate or split. DST weekends get an invite review.
None of this makes you slow. It makes you readable. People answer faster in the morning when they do not hate the badge they slept next to.
If you are about to send, and you would not ring their doorbell, do not Slack. Check London or wherever they actually are. Then schedule send.
What "async" stopped meaning
Async was supposed to mean "I write now, you read later, nobody's doorbell rings." Tools inverted it. Presence dots, badges, and threads that bump on every emoji trained people to treat timestamps as social proof of hustle. After-hours Slack is that inversion made visible.
If your team is truly async, the artifact is a document, a recorded clip, or a scheduled message. If the artifact is a mention at 22:10, you are synchronous and pretending. The pretense is what feels rude.
Test a policy for two weeks: mentions outside 09:00–18:00 local of the recipient auto-defer unless the message starts with `SEV`. Count how many "emergencies" survive the prefix. Most will wait until 09:31 and the work will improve.
Managers go first. If a director slacks at 23:00, the policy is fiction. Directors schedule send. Then everyone else can.
For a pair with a fat overlap, after-hours is almost always optional. Use New York–London in daylight. Save the night for incidents with names.