Stages of Sleep

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We generally think of sleep as a time when we relax and mentally unplug from the world. In reality, sleep is a complex process that incorporates several different stages. Each sleep stage contributes something important, and we need a balance of all the sleep stages in order to function at our best.

What Are Sleep Stages?

Sleep is generally categorized into two types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep can be further broken down into three stages, for a total of four stages of sleep, including REM sleep. Each night, we cycle through all sleep stages in order, multiple times.

Stage 1 NREM Sleep

Stage 1 sleep is the lightest sleep stage, occurring right after falling asleep and generally lasting less than five minutes. During stage 1 sleep, your brain waves, heartbeat, and breathing start to slow. Your eye movements taper down, and your muscles relax and may twitch. Brain waves during this sleep stage are known as theta waves.

Stage 2 NREM Sleep

As you enter stage 2 sleep, your heartbeat and breathing slow further and your muscles relax even more. During this sleep stage, your body temperature drops, and your eyes almost completely stop moving. The first time you enter stage 2 sleep, it lasts just under half an hour. As the night wears on, periods of stage 2 sleep become longer. Over the course of a night, we spend about half of our time in stage 2 sleep.

While brain waves are generally slower during stage 2 sleep, this sleep stage is also characterized by sleep spindles and K-complexes, short spikes in brain waves that block out external stimuli to help you sleep more soundly. K-complexes and sleep spindles also play a role in learning and memory.

Stage 3 NREM Sleep

Otherwise known as deep sleep or slow wave sleep, stage 3 sleep is the most restorative sleep stage, when heartbeat, breathing, brain waves, and muscles are at their most relaxed. It is hard to wake someone up from this state, and if they do wake up, they may feel groggy. Brain waves during deep sleep are known as delta waves.

During stage 3 sleep, the body secretes growth hormone and works to repair tissues, muscles, and bones. This sleep stage also contributes to immune system functioning and memory consolidation.

Stage 4 REM Sleep

Rapid eye movement (REM) sleep is so named because people in this sleep stage visibly move their eyes back and forth beneath their eyelids. REM sleep is very similar to wakefulness in terms of brain waves, breathing, heart rate, and blood pressure. The major difference is that during REM sleep, many of our muscles are temporarily paralyzed, which stops us from moving.

REM sleep is important for learning and memory consolidation, as well as emotion regulation. Periods of REM sleep become progressively longer with each sleep cycle during the night, starting at around 10 minutes and ultimately lasting as long as one hour.

As we sleep, we complete several cycles of all the sleep stages. It generally takes about 90 minutes to complete one entire sleep cycle. During a typical night, deep sleep periods are longer in the first half, and REM sleep periods are longer in the second half of the night. However, overall, we spend the most time in stage 2 sleep.

What Stage of Sleep Do You Dream In?

More dreaming happens during REM sleep, although it is possible to dream in other sleep stages. Dreams in REM sleep tend to be more vivid and follow more of a storyline. By contrast, dreams in other sleep stages may take on the form of disconnected thoughts and impressions.

Why Sleep Stages Are Important

Your sleep architecture is the way you progress through the sleep stages and the time you spend in each stage. Maintaining a balanced sleep architecture is important for physical and mental health.

Failing to spend enough time in each sleep stage can lead to daytime sleepiness and negatively affect growth, metabolism, and immune system functioning, as well as mental processes such as learning and memory. Going short on sleep often leads to a rebound in slow wave and REM sleep, if you have not slept long enough to accumulate significant time in these sleep stages. A rebound involves spending more time than usual in a sleep stage the next time you sleep after experiencing deprivation of this sleep stage.

What Can Interrupt Your Sleep Cycle

A number of factors can cause partial or full awakenings at night. These awakenings can interrupt the sleep cycle and lead to a higher proportion of light sleep and less time spent in restorative deep sleep or REM sleep.

Age

Infants spend much more time in REM sleep, and REM sleep continues to decrease as we age. Older adults often experience a pronounced decrease in REM sleep and slow wave sleep.

Sleep Disorders

Sleep disorders, such as sleep apnea, can cause awakenings that interfere with a person’s progression through the sleep stages. Other sleep disorders, such as narcolepsy, affect the sleep stages more directly. For example, narcolepsy causes people to enter REM sleep almost right away, bypassing the other sleep stages.

Alcohol, Medications, and Other Substances

Certain medications can suppress REM sleep. Depending on the amount, drinking alcohol before bed may increase the time spent in slow wave sleep and decrease the overall time spent in REM sleep.

Discomfort or Distractions

Chronic pain, an urge to go to the bathroom, or external factors, such as a bright light or a loud noise, may also rouse sleepers and disrupt sleep architecture.

Tips for a Healthier Sleep Cycle

Getting high-quality, uninterrupted sleep can help you attain a healthy balance of the sleep stages. Improve your sleep through sleep hygiene practices:

  • Keep a regular bedtime and set your alarm clock for the same time every day.
  • Follow a consistent, relaxing bedtime routine.
  • Aim for a comfortable bedroom environment with minimal distractions.
  • Don’t bring screens into the bedroom.
  • Exercise regularly, but not too close to bedtime.
  • Avoid alcohol, caffeine, and cigarettes in the afternoon and evening.
  • If you cannot fall asleep after a little while, go to another room and do a relaxing activity until you feel sleepy.

Talk to your doctor if you believe your sleep problems may be due to a sleep disorder or medical condition. They can help you address these issues and improve your sleep.

References

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