Abstract
Heart rate variability (HRV) measurement is an important tool in cardiac care that can provide clinicians and researchers with a 24-hour noninvasive measure of autonomic nervous system activity. Sleep and wake have profoundly different effects on HRV patterns and therefore significant implications for HRV interpretation. This article provides a brief overview of the processes underlying HRV, the standard measures of HRV, a basic overview of wake and sleep, the HRV patterns associated with different sleep and wake states, and the patterns of HRV exhibited in common cardiac conditions. The article concludes with an overview of some general health history factors that are important to consider when interpreting HRV patterns in the clinical and research setting.