What is the 90 minute sleep cycle rule?

The 90-minute sleep cycle rule is based on the idea that your body goes through several sleep stages in a cycle that lasts about 90 minutes. Waking up at the end of a cycle—rather than in the middle—can help you feel more refreshed and alert. According to this rule, you should aim to wake up at the end of a full 90-minute interval (such as after 6 hours, 7.5 hours, or 9 hours of sleep), rather than simply targeting 8 hours of sleep. The goal is quality over quantity, by aligning your wake-up time with your natural sleep rhythm.