What is a diagnostic tool commonly used in sleep medicine?

Polysomnography is a comprehensive diagnostic tool used in sleep medicine to monitor the various physiological parameters that occur during sleep. This test records brain waves, oxygen levels in the blood, heart rate, breathing, as well as eye and leg movements throughout the night, allowing healthcare providers to diagnose sleep disorders such as sleep apnea, restless legs syndrome, and narcolepsy.

The other options serve different primary purposes: an electrocardiogram measures heart activity but does not provide data on sleep patterns; ultrasound is typically used for imaging organs and soft tissues; and MRI primarily provides detailed images of the body's internal structures and is not specifically designed for assessing sleep. Thus, polysomnography is the tool specifically aimed at understanding sleep and diagnosing associated disorders, making it the appropriate answer.