The 90-Minute Sleep Cycle: Why Waking at the Right Time Matters

Your brain doesn’t sleep in one long stretch — it moves through 90-minute cycles. Wake up at the wrong point in that cycle and you can feel groggy even after a full night of sleep. In this episode we break down sleep cycles, sleep inertia, and a simple way to experiment with timing your sleep so you wake up feeling clearer. 

Transcript

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Welcome back to Night Shift with Justin S. King, How to Transform Your Life One Night at a Time. Tonight we're talking about sleep cycles. While most people think that sleep is mainly about how many hours you get, your brain doesn't actually sleep in one long stretch. It moves through repeating cycles. And each cycle lasts about 90 minutes. During that time, your brain moves through stages night sleep, deeper sleep, REM sleep, the stage where dreaming and emotional processing happen. Then the cycle starts again. Over a typical night, your brain will move through four to six of these cycles. So how you feel in the morning often has less to do with the total hours that you slept and more to do with where in the cycle you wake up. You wake up near the end of a cycle. When sleep is lighter, your brain is already moving toward wakefulness, and you'll usually feel fairly clear. If an alarm hits you while you're in deep sleep, your brain gets pulled out of the lowest point of the cycle. And that's when you will feel heavy, foggy and in a state of what researchers call sleep inertia. If you wake up during the cycle, your brain may experience grogginess for 15 minutes or even an hour. And so two people can both sleep seven hours. One wakes up clear, the other feels like they were hit by a truck. And the difference is often cycle timing. There's a simple way to think about these 90-minute blocks. For example, six hours is about four cycles, seven and a half hours will be about five cycles, and nine hours will be about six cycles. And these windows tend to line up better with the natural rhythm of sleep. And when your schedule is consistent, similar bedtime, similar wake up time, your brain begins to anticipate when you'll wake up and starts releasing signals shortly before morning. So that's why people with a stable sleep schedule sometimes wake up right before their alarm. If you want to experiment with this tonight, try something simple. Take your wake up time and count backward in 90-minute blocks. Pick bedtime that roughly lines up with one of those cycles. No need to be perfect with the math, just run the experiment for a few nights. And you may notice something subtle. You're not necessarily sleeping longer, but you are waking up feeling more refreshed. That's it for tonight's episode of Night Shift. And this is just a reminder to know what time you're going to bed each night. Keep that consistent, and that's going to have the biggest impact on consistency in your sleep cycles. If you'd like more info on that, you can head over to my website, JustinSKing.com/slash fast. Pick up a free guide that can help transform your sleep in as little as three days. My name is Justin S. King, and I help entrepreneurs find peace tonight, tomorrow, and for the rest of their lives through sleep optimization, emotion regulation, and discovering their purpose. Good night.