Like I said, I have never worked a job with fixed schedule, so I really don't know. However based on this article: https://www.historydefined.net/the-9-to-5-workday-a-brief-history/

The idea of 9 to 5 being a standard workday comes from the years just after World War II, during a high in the economy when many people worked 9 to 5 with their lunch being counted as part of the workday.

Also several of my friends work 9-5 in the U.S. including lunch, although many others work 8:30-5.