Standardized guidelines for polysomnography (PSG) have not specified methods for acquiring or interpreting electrocardiographic (ECG) data. The practice of single lead ECG monitoring during PSG may allow identification of simple measures of cardiac rhythm but reduces the ability to detect myocardial ischemia and to define cardiac intervals. Although simple measures of cardiac rhythm such as heart rate and cardiac pauses are inherently reliable, there is limited data regarding outcome measures relative to sleep related heart rates and cardiac events during sleep. Several observational and cross-sectional studies demonstrate that average heart rate drops nearly 50% from infancy through young adulthood and that the average heart rate slows during sleep compared with wakefulness; the definitions of sinus bradycardia and sinus tachycardia should therefore be lower during sleep than wakefulness. Asystoles of up to 2 seconds are seen in normal populations during sleep. Although there may be an increased risk of certain arrhythmias at night, particularly in sleep disordered breathing, there is no evidence that supports different definitions for these arrhythmias during sleep compared with wakefulness. When the quality of tracings permits, the standard definitions of narrow- and wide-complex tachycardias and atrial fibrillation may be employed. In the future, expansion to multiple ECG leads and the use of alternative tools may provide better definition of heart rates and cardiac events during sleep.