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The sleep cycle wasn't introduced to modern science until the 1950s, when researcher William C. Dement (since referred to as the father or sleep medicine) helped chart brain activity as one slept through the use of electroencephalography (EEG) testing and discovered that the brain goes through different stages as one slumbers.

The discovery negated the current theory of the 1950s, in which scientists theorized that the brain is simply "turned off" during sleep. In fact, the brain is quite active as one sleeps, as it guides the sleeper through the different stages of sleep. This discovery opened up an entire new field of scientific research, and forever changed what we know about sleep.

What Is The Sleep Cycle?
What Is The Sleep Cycle?

The sleep cycle is just that -- the stages one goes through as they sleep. A complete sleep cycle consists of five stages (Stage 1, 2, 3, 4, and REM sleep). As you sleep throughout the night, you go through the stages of sleep many times.

There are two segments of the sleep cycle, non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep progresses from a light sleep to a deep slumber, in which the breathing, heart rate and brain waves become slow, heavy and regular. REM sleep is named for when the sleeper's eyes dart around underneath the eyelids, indicating that the brain is dreaming. After REM sleep is complete, the sleeper returns to Stage 1 and repeats the process until waking.

A typical person will go through the sleep cycle three or four times a night. However, if someone is suffering from a sleep disorder, this cycle can be stalled or halted. You can read more about sleeping disorders in the article, "Sleep Disorders: What You Need To Know About The Most Common Types."

Stage 1:

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Stage 1:

Stage 1 of the sleep cycle is a light sleep, like a catnap. During this first stage of sleep, one flutters in and out of consciousness and can be easily woken. Muscle activity and eye movements begin to slow down as your brain begins to release alpha and theta waves. During this stage, muscles can suddenly contract and one can experience the sensation of falling -- think, if you've ever nodded off at your desk.

Stage 2:

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Stage 2:

This stage of sleep is also light, but not as light as Stage 1. During this stage of the sleep cycle, the brain increases bursts of wave frequency, called sleep spindles, which begin to slow down your breathing and heart rate as the body prepares for deep sleep.

Stage 3:
Stage 3:

This stage of sleep is when deep sleep begins. Though your brain is still producing little spouts of faster waves, the brain starts producing delta waves, which are extremely slow, to move you toward restoration. It is now more difficult for one in this stage of sleep to be awakened because your body is less responsive to the physical world.

It is also during this stage where one can experience sleep walking, talking in one's sleep and even sleep paralysis (the difficulty to physically move your body right after you wake up) as one begins to transition into REM sleep. You can read more about sleep paralysis in our article, "Sleep Paralysis: Everything You Need To Know About Conscious Nightmares."

Stage 4:
Stage 4:

Have you ever woken up and felt disorientated for your first few minutes of consciousness? Then you were probably woken during stage four of the sleep cycle. During this stage of sleep, your brain produces those slow delta waves almost exclusively as you approach REM sleep. This is the final stage of NREM sleep.

REM Sleep:
REM Sleep:

This is where the most intense brain activity occurs. During this stage of sleep, brain waves mimic the activity of being awake. This is the stage where eyes begin to move rapidly in different directions, blood pressure and heart rate increases, breathing becomes lighter and more irregular, and where the sleeper experiences dreaming.

The REM stage of sleep is where your brain processes information and experiences from the day before, converting it into your long-term memory. This stage of sleep begins 90 to 110 minutes into your sleep and can last for up to an hour. Your age also impacts how much time you spend in REM sleep -- infants spend up to 50 percent of their time sleeping in REM, while adults only spend about 20 percent of their sleeping time in REM.

As REM sleep is the stage where dreaming occurs, it is also the stage of sleep where you can experience lucid dreaming (where the dreamer is aware that they are in a dream and can sometimes manipulate the dream world they're experiencing). To read more about lucid dreaming, read our article, "Lucid Dreaming: Facts About The Phenomenon."

Why Is The Sleep Cycle So Important?
Why Is The Sleep Cycle So Important?

On a full night's sleep, the typical person experiences three or four complete sleep cycles. The different stages of the sleep cycle work to benefit us in varying ways. Deep sleep is essential to physical restoration and growth. According to the Harvard Business Review, without regular deep sleep one is much more susceptible to get sick, feel depressed and gain weight. REM sleep also has health benefits, but on a more psyche level. REM helps with emotions, memory, learning and improves critical thinking.

If you are having trouble sleeping soundly through the night or getting to sleep on time, read some tips for a better sleep and how to fall asleep fast here.

How Is The Sleep Cycle Studied?
How Is The Sleep Cycle Studied?

Since the mid-twentieth century, the sleep cycle has been studied through electroencephalography (EEG) testing. EEG test tracks and records brain waves and patterns through electrodes attached to the subject's scalp.

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