About 15 million Americans don’t have a typical nine-to-five workday, and many of these — nurses, firefighters and flight attendants, among many other professions — may see their schedule change drastically one week to the next. As a result, these shift workers’ biological clocks, which keep track of the time of day, cannot keep accurate time, potentially making the negative effects of a high fat diet on metabolic disorders even more pronounced, according to new research.