Which device would a sleep researcher use to monitor a sleeping person's brain waves?

A sleep researcher will use EEG to monitor a sleeping person's brain waves.

Explanation:

EEG stands for 'Electroencephalogram'. The EEG test is done to track and record brain wave patterns. Electrodes are placed on a person's brain scalp which detects brain signals and send it to a computer which shows the brain waves pattern. A researcher or a doctor can compare it to the pattern of waves of a normal brain and look for abnormalities to detect a special condition or disease.

EEG is used to find problems related to electrical activity of the brain. It is usually done to diagnose and monitor seizure disorders. Also, to observe brain activity in case of sleeping disorders or change in behavior.