During sleep studies, what does EMG primarily measure?

EMG, or electromyography, is primarily used to measure muscle tone during sleep studies. It specifically assesses the electrical activity of skeletal muscles, providing insights into muscle function and activity levels. In the context of sleep studies, EMG data is crucial for evaluating conditions such as REM sleep atonia, which involves a significant reduction in muscle tone. This relationship between EMG signals and muscle tone helps differentiate between various sleep stages, particularly identifying REM sleep, where muscle tone is typically at its lowest.

The other options pertain to different physiological measurements: heart rate variability is tracked through ECG (electrocardiography) methods, eye movement is recorded using EOG (electrooculography), and brain wave patterns are monitored using EEG (electroencephalography). Each of these techniques serves specific purposes in sleep studies, but EMG is uniquely focused on assessing muscle activity, solidifying its role in understanding muscular responses during sleep.