Categories: Sleep Science

Sleep Stages and Sleep Cycles

Long ago, it was belived, that when people fall asleep, their bodies and brains enter a passive state, and shut down to provide rejuvenation from the day. Since then, researchers have learned that sleep is more complex. A sleeper’s body and brain actually go into an “active” state. In fact, a sleeper’s body and brain goes through a variety of activities while sleeping, known as Sleep Stages and Sleep Cycles.

Understanding the 5 Sleep Stages

While sleeping, a lot occurs in your body and you cycle between non-REM and REM sleep. You first drift into non-REM sleep, and after a short period, into REM sleep, and then start the cycle over again. Each sleep stage typically lasts 90 minutes, with each stage lasting up to 15 minutes.

Stage One

Within a few minutes of nodding off, your brain begins producing alpha and theta waves with your eye movements slowing down. This is a relatively brief introduction to sleep, and lasts up to seven minutes. You’re in a light sleep stage, meaning you’re still alert, and may easily be woken. In this sleep stage, sleepers often indulge in short “catnaps.”

Stage Two

In this sleep stage, you’re also engaging in light sleep, and your brain is producing sudden brainwave frequency increases, referred to as sleep spindles. Your brain waves start slowing down. If you’re indulging in a “power nap,” it’s after this sleep stage that you would want to wake up.

Stage Three

When entering stage three, extremely slow brain waves (delta waves) are scattered with smaller, faster waves. You’re in a deep sleep at this stage, and this is where you could experience things like:

  • Night terrors
  • Sleepwalking
  • Bedwetting
  • Talking while sleeping

These behaviors are referred to as parasomnias, and often occur while transitioning between non-REM and REM sleep.

Stage Four

Deep sleep continues in this stage as your brain continues producing delta waves almost exclusively. Individuals awakened during this stage feel disoriented for several minutes.

Stage Five (REM sleep)

Generally, you enter the REM sleep stage around 90 minutes after falling asleep initially, and each REM stage may last up to an hour. The average adult experiences five to six REM cycles every night. In this final stage of sleep, your brain is becoming more active. You dream during this stage, and your eyes are jerking rapidly in different directions. Also, your:

  • Breathing becomes irregular, fast, and shallow
  • Heart rate and blood pressure rise

REM sleep also plays a significant role in memory function and learning, since this is the stage where your brain is consolidating and processing information from the previous day to store for your long-term memory.

Understanding Sleep Cycles

A sleep cycle is a period of time individuals take to progress through these stages above. But, people don’t go from deep sleep straight to REM sleep. Instead, they progress into a sleep cycle through non-REM sleep stages going from light sleep to deep sleep, and then reversing back from deep to light sleep. Then, they end with REM sleep time, before they start the cycle over, in light sleep once again. For instance, the cycle order may look something like this:

Light sleep (stage 1), to light stage (stage 2), to deep sleep (stage 3), to light sleep (stage 2), to light sleep (stage 1), to REM sleep.

Let’s take a look at each cycle a little further:

Stage 0 (Waking): Your eyes open and are responsive to external stimuli. You’re able to hold intelligible conversation.

Stage 1 (Light sleep): You’re transitioning between waking and sleeping. If awakened, you will feel like you never slept.

Stage 2 (Light sleep): This is the stage of light sleep. This is the synaptic pruning and memory consolidation stage.

Stage 3 (Deep sleep): EEG readings show slow waves.

Stage 4 (Deep sleep): EEG readings show slow waves.

Stage 5 (REM sleep): Your brain waves are similar to waking, and you usually have vivid dreams during this stage. Your body doesn’t move.

How Long is a Sleep Cycle?

Your first cycle of sleep takes around 90 minutes. Then, the cycles average around 100 to 120 minutes. Usually, you’ll go through four to five sleep cycles per night.

Does Your Sleep Cycle Change With Age?

The various sleep cycles last for different time periods each night. You’re in non-REM sleep stage most of the first part of the night and REM sleep stage increases during the second part of the night.

For instance, the sleep cycle of an infant will be different than the sleep cycle of an adult. Infants spend nearly 50 percent of their sleep time in REM sleep, whereas adults are spending nearly 50 percent of their sleep time in stage two, and only 20 percent in REM and the rest of the time divided between the other stages. The elderly spend progressively less of their sleep time in REM sleep.

Understanding the different sleep stages, how you cycle through them, and the importance of quality sleep hours, is valuable information for all people, as they strive to gain more knowledge about their own sleep health.

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    Amber Merton

    Amber Merton is an accomplished writer on the topics of green living and sleep. Her work has been covered in numerous online publications. Amber has been a regular author on the PlushBeds blog for the past 7 years.

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