What Is a Sleep Cycle?
A sleep cycle is a repeating sequence of sleep stages — light sleep, deep sleep, and REM (rapid eye movement) sleep — that the brain moves through several times each night. One cycle lasts roughly 90 minutes on average, with a typical range of about 70 to 120 minutes.
How Cycles Change Overnight
Earlier cycles tend to contain more deep sleep, and later cycles more REM sleep. A full night usually includes four to six cycles.
Waking Between Cycles
Being woken from deep sleep can cause grogginess known as sleep inertia, which is why some people time alarms to the end of a cycle.
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🔗 Related Terms
More Health & Fitness Terms
- Basal Metabolic Rate (BMR)
- Body Composition
- Body Fat Percentage
- Body Mass Index (BMI)
- Calorie
- Calorie Deficit
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Last updated: September 27, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.