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Sleep Tips

All About Sleep Stages and REM Sleep


Why do we sleep in "cycles"? And, how long is a sleep cycle, and what is REM sleep? While sleep may seem like a simple human need, it is actually a pretty complex process. There is very nuanced and complex science for what happens during a seemingly simple night's rest.

So, if you've ever wondered what happens to your body during sleep, I'm here to help resolve some of your confusion. Here's what you need to know about the optimal sleep cycle:

What are sleep cycles?

First, it's important to understand that sleep cycles are very different from stages of sleep. While most people use "stages of sleep" and "sleep cycle" interchangeably, the two aren't the same. There are actually various differences:

  • A sleep cycle is a period when our brains move through the different stages of sleep.
  • A sleep cycle can last anywhere between 90 to 120 minutes.
  • Most humans have 4-5 sleep cycles in a night.
  • Each sleep cycle contains four stages of sleep.

What are the stages of sleep?

There are two types of sleep: NREM (non-rapid eye movement) and REM (rapid eye movement) sleep.

The first three stages of a sleep cycle are NREM sleep, and these eventually broaden into the fourth and final phase where REM sleep occurs. During the sleep cycle, humans move through the first three stages of sleep, then reverse back through the stages (from Stage 3 to Stage 1) before entering the fourth stage of sleep.

  • Stage 1: Duration: 5-10 minutes. In stage one humans experience very light sleep, plus a few muscle twitches and deepening of the breath. According to, our brain activity slows by 50% during stage 1.
  • Stage 2: Duration: 5-15 minutes. During stage 2 your body temperature dips and your breathing slows even further. The largest portion of each night's sleep occurs during Stage 2 (up to 55%!).
  • Stage 3: Duration: 45-90 minutes. This is the most restorative of sleep stages where your body repairs itself and prepares for the next day. It's hard to wake someone from this sleep, and this “good sleep" happens more often in the first half of the night.
  • Stage 4: Duration: 10-60 minutes. REM sleep begins, and vivid dreaming occurs. Scientists theorize that during this sleep stage your brain makes the neural connections necessary to learn and retain information.

Does dreaming really happen during REM sleep?

Yes, dreaming happens during REM sleep. But just because you don't remember dreaming, doesn't mean it didn't happen. Whether or not you remember your dreams depends on a variety of factors, some good and some bad:

  • Individual brain activity
  • How often you wake in the night
  • Use of alcohol or drugs prior to bedtime

While it is possible to dream in the other sleep stages, they're just not as vivid as the dreams had during REM sleep.

Why is REM sleep so important?

Something extra special happens during REM sleep. Essentially, our brain imitates how active it is when we are waking, but our muscles are paralyzed. This phenomenon keeps us from acting out our dreams (like ballroom dancing in the living room like we're on Dancing with the Stars or actually starting the car to go out on a road trip with Marilyn Monroe.)

Not only is REM sleep shown to improve memory, learning and cognition, it is theorized to be the neural pathway toward overall health and well being. Researchers don't quite know WHY we need REM sleep, or really why we dream at all, but what we do know is that humans are likely to die without it.

In a study where rats were continually deprived of REM sleep their lifespans were dramatically shortened.

What happens if I don't get enough REM sleep?

Because REM sleep happens in the last third of a night of sleep, many awake from REM sleep simply because it is time to get up in the morning. This is why some days you may feel groggier than others.

Still, the symptoms of immediate lack of REM sleep often mirror what happens to your body after a night of poor sleep: irritability, fatigue, inability to concentrate, poor memory, etc. So, in the long term, lack of REM sleep may lead to increased risk of obesity, long-term memory loss, sleep apnea, cardiovascular disease and even diabetes.

As if you need another reason to get a full, deep night of sleep, it turns out your life can depend on it.

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