[LeapingLennie]

Yes, I've also had managers that start their day at some ungodly hour in the morning and so it wouldn't be unusual to receive emails from them at 5am. There was never any expectation to respond to emails straight away and the same should apply to emails sent in the evening.

[SAI_Peregrinus]

One nice feature of Slack is that you can schedule messages to send when the recipient's working hours start. Let the automation take care of keeping messages to working hours.