Circadian rhythms versus daily patterns in human physiology and behavior

The endogenous circadian timekeeping system modulates human physiology and behavior with a near 24 h periodicity conferring adaptation to the ~24 h solar light-dark cycle. Thus, the circadian timekeeping system times physiology and behavior so that it is prepared for environmental changes. The term circadian implies an endogenous “clock-driven” process. However, not all observed daily patterns in physiology and behavior are clock driven and instead may be due to environmental or behavioral factors. For example, the barren rock on the top of a mountain shows a daily temperature oscillation that is not endogenous to the rock but instead is caused by the sun heating the rock during the day and radiative heat loss after sunset. Other factors such as wind, rain, and cloud cover impact the observed daily temperature oscillation of the rock. Similarly, some of the daily patterns observed in physiology and behavior are driven by external factors, while others arise from the interaction between circadian and behavioral processes (e.g., sleep-wake, fasting-feeding). To improve understanding of the mechanisms underlying observed daily patterns in physiology and behavior in humans, a variety of circadian protocols have been implemented (Tables 13.1 and 13.2). These protocols will be reviewed in the following pages, and the strengths and limitations of each will be discussed. First, we review markers of the endogenous clock in humans.