Waiting for a reply creates a day partly organized by someone who may not know it.

Waiting on a reply hands a portion of your day to someone who has no idea they are holding it. You check, you half start things, you keep an interior door open, and the shape of the hours bends around a message that may not even be near the top of their list. The asymmetry is not unkindness on their part; it is simply that your attention is where theirs is not.

When you notice a day organising itself around a pending answer, put a shape on it yourself. Decide when you will check, and what you will do with the day if nothing comes. And when someone is waiting on you, a short holding note costs almost nothing and gives them their day back.