An in-laboratory sleep study attaches an array of sensors to a patient for a single night. Each channel exists because a specific clinical question requires information that no other channel provides.
Sleep stages need three signal types
Scoring a night into stages requires brain activity from electroencephalography, eye movement from electrooculography, and muscle tone from chin electromyography, read together.
Rapid eye movement sleep is identified by the combination of a low-amplitude mixed-frequency brain pattern, characteristic eye movements, and a striking drop in chin muscle tone.
No single one of those establishes the stage. The definitions are explicitly combinatorial, which is why the sensor array cannot be trimmed down.
Breathing is measured in several places at once
Airflow is recorded at the nose and mouth, while separate belts around the chest and abdomen record respiratory effort.
The distinction matters because an event with absent airflow but continuing effort indicates an obstruction, while absent airflow with absent effort points to a central signaling problem.
Oxygen saturation from a fingertip sensor records the consequence, and the timing between the breathing event and the saturation drop is part of the interpretation.
Events are scored against defined rules
A technologist reviews the recording in short epochs and applies published scoring criteria for what counts as an apnea, a hypopnea or an arousal.
Those criteria specify duration thresholds and the size of the required change, which is what makes results comparable between laboratories.
Because the definitions have been revised over time, an index from an older study is not always directly comparable to a recent one.
Movement and position are recorded too
Leg electrodes capture periodic limb movements, which can fragment sleep independently of breathing.
A position sensor tracks whether the patient is supine, since airway collapsibility often changes with body position and the pattern influences what is recommended afterward.
Audio and video recording provides context for unusual behavior during the night that the electrical signals alone would not explain.
Why home testing is not always equivalent
Home sleep apnea tests record a reduced set of channels, typically airflow, effort and oxygen, without brain activity.
Without electroencephalography, total sleep time is estimated rather than measured, and arousals cannot be scored, so indices are calculated against recording time instead.
Which test suits a given person depends on the suspected condition and other medical factors, a decision made by a sleep physician rather than chosen from a menu.