If you spend time calling on people, you know why this is something you need to think about. You can save a lot of time by calling on people when they're available and in a mood to talk rather than when they're busy.

It's professional and polite to try to call on people at times that are appropriate for them. You're doing them a courtesy, even if they don't realize it. Doing so makes you stand out from the crowd of everyone else who ignores this.

For a short time there was a salesperson in our town who called on restaurants. Most days he made his calls when the restaurant owners and managers were preparing for lunch or dinner. He never seemed to figure out that restaurants, and their managers, tend to be very busy during certain hours. He didn