How effective medications are depends on various factors, including the time of day when they are administered. Why? Because our bodies don't always function exactly the same. Instead, they follow the cycle set by their internal clock, otherwise known as circadian rhythm. But since each person's circadian rhythm is different and depends on a number of different factors, it is difficult to tailor medication schedules to an individual patient's body clock. Researchers have now developed a method for determining the optimum time of cancer treatment based on certain breast cancer cell lines.