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Sleep Cycle

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The sleep cycle is a repeating process that people go through every night. The full cycle lasts around 90 minutes, and a full night of sleep normally has four or five cycles, each made of four stages.

What Is The Sleep Cycle?

The sleep cycle is a series of four separate sleep stages throughout a person’s nightly sleep. Sleep stages correspond to the “depth” of sleep, and each stage is responsible for different aspects of rest.

The sleep cycle begins with the transition from wakefulness to sleep and ends with a return from deep to shallow sleep and, eventually, wakefulness.

Why Sleep Cycles Matter

Each component of the sleep cycle affects different aspects of rest and rejuvenation. Sleep cycles are crucial in maintaining overall sleep quality and optimizing brain and body function.

Sleep is a complex process, but understanding the non-REM and REM sleep cycles can help one build good sleep habits, negate excessive daytime sleepiness, and improve sleep quality regardless of sleep quantity.

Getting enough total sleep is important, but getting enough of the right kinds of sleep matters, too.

The Circadian Rhythm

The circadian rhythm is the body’s biological clock. Influenced by environmental and hormonal processes, this 24-hour cycle tells the body when to get tired, fall asleep, and wake up based on internal and external factors.

The circadian rhythm governs sleep patterns, meaning a properly-adjusted circadian rhythm is needed to start and end each sleep cycle at appropriate times.

Components of a Good Night’s Sleep

High-quality sleep has a few important parts. Most people talk about getting enough sleep, but sleep quality has more components than sleep duration.

The total number of sleep cycles, time spent in each stage of the sleep cycle (N1 and N2, N3, and REM sleep) and the stage when someone wakes all play into overall sleep quality.

Naps and short-duration sleep have different goals than sleeping throughout the night - a light sleep typically goes no lower than Non-REM 1 or 2, making it easy to awake and feel refreshed without completing a full sleep cycle.

A good night’s sleep, though, normally requires four or five full sleep cycles of 90-120 minutes each. Early sleep cycles tend to be shorter, with the cycles just before waking running longer.

The proportion of time spent in deep or REM sleep also increases with subsequent sleep cycles during the night. The first sleep cycles tend to have less deep sleep, with more time spent in NREM-3 and REM sleep in later hours.

Sleep Stages vs. Sleep Cycles

Sleep physiology is an extremely complex science, and the difference between sleep stages and sleep cycles can be confusing. In simplest terms, the sleep cycle is the process of all four of the sleep stages running together.

One sleep cycle includes the four sleep stages. Going to bed and falling into the first sleep stage, Non-REM 1 Sleep, passing deeper to NREM-2 and 3, then into and through rapid eye movement (REM) sleep, completes the first sleep cycle.

Each normal sleep cycle comprises four sleep stages, and sleeping throughout the night typically includes four or five sleep cycles.

Non-REM Sleep

Sleep is broadly classified as either rapid eye movement sleep (REM) or Non-Rapid Eye Movement (NREM) sleep. Non-REM sleep occurs earlier in the sleep cycle and is split into three NREM sleep stages: Non-REM 1, Non-REM 2, and Non-REM 3 (also known as deep sleep or slow wave sleep).

NREM-1: The Transition

The first NREM sleep stage involves the transition from wakefulness to sleep. Reduced muscle activity, slow eye movements, and a lack of awareness of surroundings characterize this light sleep.

Brain activity begins to drop during NREM-1 as the frequency of brain waves decreases compared to wakefulness. NREM-1 is the “power nap” sleep stage, and brief periods of sleep that do not go deeper than N1 normally leave someone feeling refreshed but not very rested.

NREM-2

NREM Stage 2 is deeper than Stage 1 and is observed when brain waves slow further. Heart rate reduces, body temperature drops, and sleep spindles are observed. Despite the decreased brain activity, occasional bursts called sleep spindles are seen.

Sleep spindles are not well-understood, but some sleep researchers believe they play a role in memory consolidation and neuroplasticity - vital components in learning. NREM-2 sleep also restores energy.

Waking during NREM-2 can leave someone feeling groggy and disoriented and generally not as refreshed as waking from lighter sleep.

NREM-3: Deep Sleep

NREM-3 is also known as deep sleep, delta wave sleep, or slow wave sleep because of the extremely low-frequency delta waves emitted by the brain during this stage. Delta waves are the lowest-frequency brain waves emitted during sleep, indicating very little brain activity.

NREM-3 is known as restorative sleep and is the sleep stage most commonly associated with growth and development, tissue repair, strengthening of the immune system, and healing of wounds.

NREM-3 is the deepest stage of Non-REM sleep and is closely related to physical and mental health. Waking during this deep, non-REM sleep stage often leads to long-lasting grogginess and disorientation or sleep inertia.

REM Sleep

REM sleep is the deepest stage of sleep. REM stands for “Rapid Eye Movement,” observed during REM sleep. REM is characterized by decreased body temperature, irregular breathing, decreases in muscle tone, and an increased heart rate.

Unlike other stages of sleep, brain activity spikes during REM sleep. Along with increased brain activity, the eyes move quickly behind the eyelids. Researchers are still investigating the exact mechanism behind REM sleep and eye movement.

Some researchers believe that brain activity related to dreams can activate the parts of the brain responsible for vision, causing the eyes to move.

Dreaming

REM Sleep is the stage of dreams. During REM sleep, brain activity skyrockets, and some of this activity is due to dreaming.

The cerebral cortex, responsible for many higher-level and executive functions in the brain and the perception of feelings in the body, shows high activity levels during REM sleep, rivalling waking brain activity.

Interestingly, the prefrontal cortex, which is responsible for logical decision-making and self-awareness, is impeded. This combination might explain why dreams can feel so real while being entirely nonsensical.

REM and The Nervous System

Waking from a deep sleep with sweaty palms and a pounding heart can be a disturbing experience, but it might be normal: researchers have found a link between REM sleep and sympathetic nervous system activation.

The sympathetic nervous system is responsible for the Fight or Flight response and primes the body for action. During REM sleep, researchers found increased activation of the sympathetic nervous system, possibly related to dreaming, leading to adrenaline spikes, sweating, and irregular breathing.

Why REM Sleep is Important

Like some stages of NREM sleep, REM is involved in learning and memory consolidation. REM’s impact on memory is very pronounced: people who are REM sleep deprived often exhibit symptoms of dementia, with reduced cognitive ability and poor memory recall.

A research study done for the Canadian Sleep and Circadian Network compared predictive factors for dementia to obstructive sleep apnea symptoms.

Researchers found a strong correlation between sleep apnea and dementia in older adults, possibly indicating the importance of REM sleep for brain function in aging.

Sleep apnea occurs when oxygen flow is restricted, causing people to wake frequently at night. This sleep disorder prevents people from reaching the deeper stages of sleep.

Beyond cognitive power, REM sleep plays an important role in emotional regulation. People experiencing sleep deprivation, especially REM sleep deprivation, are much more likely to experience mental and emotional wellness issues than those with more hours of deeper sleep.

Waking Up

Waking up should naturally happen at the tail end of REM sleep. As one normal sleep cycle ends with REM sleep, another begins with NREM-1. This is the shallowest stage of sleep, and it is in those transitionary moments that a person will naturally wake up if their circadian rhythm dictates.  

Sleep Inertia

Sleep inertia is a condition of extended drowsiness following waking up. It can be caused by being awoken during a deep sleep stage like NREM-3. Waking during very deep sleep can prolong feelings of tiredness and cause disorientation, confusion, and fatigue.

Frequently Asked Questions about Sleep Cycle

How much sleep do I need? How much slow wave sleep, and how much REM sleep?

Getting enough sleep is important, but that amount is different for everyone. As long as you fall asleep at a reasonable time and get enough total sleep, your biological clock will self-govern how much time it spends in each of the stages of sleep.

If you feel like you’ve had a poor sleep, even if you’ve had eight or more hours of sleep, that may be evidence of a sleep disorder and it would be worthwhile to speak to your doctor.  

I have trouble falling asleep. What should I do?

Trouble falling asleep is quite common, with about 20% of Americans experiencing issues falling asleep. People often turn to sleep medicine, but there are better choices than this.

Sleep medicine can disrupt the stages of sleep, reducing restorative sleep and disrupting the REM period. Practicing some sleep hygiene improvements, meditating, breathing slow, and keeping your room cold are natural remedies that can help with falling asleep.

How do sleep disorders impact the sleep cycle?

Sleep disorders can have serious impacts on the sleep cycle. Sleep apnea, for instance, can prevent someone from reaching deeper sleep stages for years.

People with narcolepsy, a condition characterized by random sleep attacks during wakefulness, can fall through NREM sleep and into REM sleep within 15 minutes.

Because each of the stages of sleep affects different aspects of rest, understanding sleep stages and sleep patterns can help identify and treat sleep disorders.  

I got so much sleep! Why am I still tired?

Subjective sleep quality isn’t always a fair indicator of restful sleep. Just because you woke up feeling tired, it doesn’t mean you got a poor sleep. You could still be tired after getting lots of sleep for many reasons.

The culprit could be one of several sleep disorders; it could be sleep inertia caused by waking during a deeper sleep stage, sleep apnea preventing deeper sleep, or issues with sleep continuity or staying asleep.

Why is sleep so important?

Sleep influences every aspect of human health. Getting enough sleep improves the immune system, lowers blood pressure, increases the release of healthy and stress-reducing hormones, and more.

Each sleep stage improves a different part of health, and understanding sleep as a component of mental, physical, and emotional wellness can help improve holistic wellness.

References

Physiology of Sleep | Concise Medical Knowledge 

What are Sleep Spindles? 

Sympathetic-Nerve Activity During Sleep in Normal Subjects 

What is REM Sleep and How Much Do You Need? 

Is it dementia or the effects of sleep apnea? 

Biomarkers of dementia in obstructive sleep apnea - ScienceDirect 

What Are Sleep Deprivation and Deficiency? | NHLBI, NIH 

REM Behavior Disorder | Stanford Health Care 

Sleep cycle - Wikipedia 

Disclaimer

The contents of this article are provided for informational purposes only and are not intended to substitute for professional medical advice, diagnosis, or treatment. It is always recommended to consult with a qualified healthcare provider before making any health-related changes or if you have any questions or concerns about your health. Anahana is not liable for any errors, omissions, or consequences that may occur from the use of the information provided.

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