The diner works using three waitresses all of whom start at the exact same time of day. If Mel's is open early for breakfast and stays open to serve dinner that means that all three waitresses work 10-12 hour days, at least SIX days out of the week.
Even for a 1970s restaurant those types of hours would be excessive. Yet none of the waitresses complain about it in more than a comedic fashion.
Phoenix, Arizona has some of the highest mean temperatures in the United States. Despite the series being set there, NONE of supporting characters are shown wearing short pants and the cast often dresses as if they were in a more temperate climate even in the middle of summer.