A term can mean something slightly different in a consumer app and a medical report. These definitions give you a starting point; ask the person who ordered a test to explain your own results.
Stages and timing
REM sleep
Rapid eye movement sleep, a stage associated with vivid dreaming. It cycles with non-REM sleep through the night. Read sleep basics.
Non-REM sleep
The group of sleep stages outside REM, including lighter and deep sleep. A clinical study identifies them from brain activity and other signals.
Deep sleep
A non-REM stage also called slow-wave sleep. A watch's deep-sleep number is an estimate, not a direct brain-wave measurement.
Core sleep
Apple's label for lighter non-REM sleep in its stage display. It is a normal part of the night. Understand the watch report.
Circadian rhythm
An approximately 24-hour pattern that helps coordinate sleepiness and alertness. Light is an important timing cue. Read the body-clock guide.
Tracking terms
Time in bed
The interval you spend in bed. It can include time awake as well as time asleep.
Total sleep time
An estimate of the time you actually slept during a sleep period. A wearable and a sleep study may calculate it differently.
Sleep efficiency
Total sleep time divided by time in bed, expressed as a percentage. A single number does not explain why you were awake.
Sleep score
A summary calculated by a device or app from selected measurements. Scores differ by product and are not diagnoses.
Sleep diary
Your own record of approximate sleep times, awakenings, daytime alertness, and relevant habits. Print a simple diary.
Medical testing terms
Polysomnography (PSG)
An overnight medical sleep study that records multiple signals, including brain activity, breathing, and oxygen levels. Learn about sleep studies.
Home sleep apnea test (HSAT)
A prescribed medical test performed at home to evaluate suspected obstructive sleep apnea in selected people. It is different from a consumer watch.
Apnea-hypopnea index (AHI)
A measure of breathing events per hour used in sleep apnea evaluation. The clinician interprets it alongside symptoms and the type of test.
Conditions and care
Insomnia
Recurring difficulty falling asleep, staying asleep, or waking too early, with effects during the day. Read insomnia basics.
Obstructive sleep apnea
Repeated narrowing or blockage of the upper airway during sleep, causing breathing interruptions. Read sleep apnea basics.
CBT-I
Cognitive behavioral therapy for insomnia: a structured treatment that addresses sleep-related behaviors and thoughts. It is more than general bedtime tips.
Stimulus control
A CBT-I approach that helps reconnect bed with sleep, including leaving bed when you cannot sleep and returning when sleepy. A clinician can tailor it to you.
One useful rule: A term on a report describes a measurement or estimate. It does not tell the whole story of your health. Bring persistent symptoms to a qualified healthcare professional.