Which of the following is a common tool used for sleep monitoring?

A polygraph, often used in the context of polysomnography, is a common tool for sleep monitoring. It records various physiological parameters during sleep, including brain waves (via EEG), eye movements (EOG), muscle activity (EMG), heart rate, and oxygen saturation levels. This comprehensive data collection allows healthcare professionals to assess different sleep stages and diagnose sleep disorders effectively.

The other tools listed do not primarily focus on monitoring sleep. An electrocardiogram (ECG) is used to measure heart activity, rather than the overall physiological changes during sleep. A spirometer measures respiratory function and is primarily used in pulmonary assessments. A photoplethysmogram assesses blood flow and is typically utilized for measuring heart rate and blood oxygen saturation but does not provide a full picture of sleep stages and qualities as a polygraph does. Thus, the polygraph is the most relevant tool for comprehensive sleep monitoring.