It's partially discipline but culture plays a role as well. If everyone at your company is sending slack messages at 7 and you aren't it becomes easy to be branded not a team player.
Which is not a bad thing in my opinion. :-)
As they say in Germany: "Ist der Ruf erst ruiniert, lebt es sich ganz ungeniert." (When the reputation is only ruined, one lives pretty unbothered).
If you struggle with your branding, then manage your brand in a way that doesn't result in killing yourself with anxiety of missing out.