A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the ＿＿（16）＿＿ of countries and their subdivisions because it is convenient for areas in close communication and interaction to keep the same time. From east to west, four time zones, ＿＿（17）＿＿ , Eastern, Central, Mountain, and Pacific, spread across the continental United States. Noticeably, a recent study found that living on the “wrong” side of a time zone can be ＿＿（18）＿＿ to one’s health. The researchers tracked about one million people’s sleep times. They found that bedtimes shift from east to west, with a sharp ＿＿（19）＿＿ happening once one crosses into a new time zone. The key to understanding the finding is knowing the link between time zones and local sunset times. Traveling east to west, one can find sunrise and sunset times get later. In general, people on the late sunset side of a time zone are more likely to be sleep ＿＿（20）＿＿ , more likely to sleep less than 6 hours, and less likely to sleep at least 8 hours. Given what is known about the importance of sleep for good physical and mental health, it is no surprise that a greater incidence of health problems, including overweight, diabetes, heart attack, and breast cancer, was found in areas where the sun sets later.