The sun both rises and sets later in the western parts of time zones, by about an hour at the extremes. If sunrise in Nashville, Tennessee is 6:30 a.m., it's about 7:30 a.m. in Amarillo, Texas. Similarly, if sunset in Nashville is 8 p.m., it's about 9 p.m. in Amarillo. Yet both are in the Central time zone.
This means that many people in Amarillo must get up before sunrise to get to work on time compared to people in Nashville, who can get up after sunrise. As I reported previously, two studies have shown that breast cancer risk is higher in the western parts of time zones; the first was conducted in European Russia and the second in the U.S. The authors of these two studies suggest it is the chronic disruption of circadian rhythms from getting up in the dark that may be the culprit.