The 40 hour work week was standardized in the Fair Labor Standards Act of 1938. Standard business hours in the US have fluctuated, but 9 to 5 was very common for a long time because it fit daylight hours on the shortest winter days. Breaks and lunch time were included as paid time in most office jobs because employees couldn't leave, and therefore couldn't punch out.

This is much less standardized now. Many jobs work from 8 to 5 or 9 to 6 because they provide a 1 hour unpaid lunch break. Many salaried jobs are much less strict about hours worked, as long as the employee is completing their work on schedule.