A sleep study usually includes all but which of the following measurements?

In a typical sleep study, various measurements are taken to assess different aspects of sleep architecture and physiological functions. EOG (electrooculogram), which records eye movements, is crucial for identifying sleep stages, especially REM sleep. EMG (electromyogram) measures muscle activity, which can help in determining sleep disorders such as REM sleep behavior disorder. Respiratory effort monitoring is vital for diagnosing sleep apnea and related disorders by tracking chest and abdominal movements.

ERG (electroretinogram), however, focuses specifically on measuring the electrical responses of the retina to light stimuli. This measurement is not relevant to the analysis of sleep stages or disorders, making it unnecessary in the context of a typical sleep study. Hence, it is not included among the standard measurements taken during this type of assessment.