CLOCK and cancer: However, there are two problems with using it as a marker of circadian phase for chronotherapy: (i) determining DLMO requires multiple consecutive samples of blood or saliva collected across an 8- to 24-hour interval under controlled conditions and (ii) DLMO represents the central clock time (“phase”) [14], which differs from the circadian clock time in peripheral tissues [15] that are vulnerable to the damage of cancer therapy (both chemotherapy and radiation).