Stages of Sleep: The Sleep Cycle

There are five stages of sleep during the sleep cycle. Scientists categorized the stages of sleep based on the characteristics of the brain and body during sleep. Stage 1,2,3, and 4, are categorized as ‘non-REM sleep’, and the fifth stage, is REM sleep. Generally, brainwave frequencies and amplitudes from an electroencephelogram (EEG)  are used to differentiate the different stages of sleep, along with other biologic rhythms including eye movements (EOG) and muscle movements (EMG).

Non-REM Sleep of the Sleep Cycle

Stage 1 Sleep

Stage 1 of the sleep cycle is the lightest stage of sleep. The EEG brain frequency is slightly slower than during wake time. There is muscle tone present in the skeletal muscles. Breathing occurs at a regular rate.

Stage 2 Sleep

Stage 2 usually follow Stage 1 and represents deeper sleep. During Stage 2 sleep, the sleeper is less able to be awakened. Stage 2 sleep is characterized by ‘saw tooth waves’ and sleep spindles.

Stage 3 and 4 Sleep – Deep Sleep

Stage 3 and Stage 4 sleep of the sleep cycle are progressively deeper stages of sleep. These stages of sleep are also called ‘Slow Wave Sleep’ (SWS), or delta sleep. During SWS, the EEG shows a much slower frequency with high amplitude signals (delta waves). A sleeper in SWS is often difficult to awaken. Some studies have demonstrated that very loud noises, sometimes over 100 decibels, will not awaken some during SWS. As humans get older they spend less time in slow wave deep sleep and more time in Stage 2 sleep.

Slow-wave sleep is generally referred to as deep sleep, and is comprised of the deepest stage of NREM. In stage three we see the greatest arousal thresholds, such as difficulty in awakening, and so on. After being awoken, the person will generally feel quite groggy, and cognitive tests that have been administered after being awoken from the third stage show that for up to half an hour or so, and when compared to awakenings from the other stages, mental performance is moderately impaired. This is a phenomenon known as sleep inertia. When sleep deprivation has occurred there’s generally a sharp rebound of slow-wave sleep, which suggests that there’s a need for slow-wave sleep. It now appears that slow-wave sleep is a highly active state, and not a brain quiescence as previously believed. In fact, brain imaging data shows that regional brain activity during non-REM sleep is influenced by the most recent waking experience

Stage 5 Sleep (REM Sleep) of the Sleep Cycle

Stage 5 of the sleep cycle, or REM sleep, is the stage of sleep associated with dreaming. It is very different physiologically from the other stages of sleep. The EEG resembles wake time. However, the skeletal muscles are atonic, or without movement. The breathing is more erratic and irregular. The heart rate often increases. It is theorized that muscle atonia evolved in order to protect the individual from injury during sleep.

sleep study data showing sleep stages

K-Complexes and Sleep Spindles

Spindle activity is exclusive to NREM sleep, with most occurring at the start and end of NREM. Sleep spindles engage brain activation in the superior temporal gyri, anterior cingulate, insular cortices and the thalamus. Sleep spindles have different lengths; with slow spindles associated with an increase in activity in the area known as the superior frontal gyrus ranging between 11 and 13 Hz, and fast spindles associated with the recruitment of both the hippocampus and the mesial frontal cortex and the sensorimotor processing cortical regions ranging between 13 and 15 Hz. At this point in time it’s not clear what is meant by these sleep spindles, but it’s hoped that ongoing research will reveal their function.

Defining K-Complexes

K-Complexes are also exclusive to NREM sleep, and can be defined as single long delta waves lasting for only a second. Like sleep spindles, they appear automatically during the early stages of sleep, generally in stage two. However, K-Complexes can be induced at will by momentary noises, such as someone knocking on a door. Further research must be conducted on K-Complexes because their function is currently unknown.

Dreaming during NREM

During REM sleep, study participants reported both intense dream vividness and improved memory of dreams which occurred during that phase, which suggests that dreaming typically occurs REM sleep: we know that dreaming also occurs during NREM sleep; however, in comparison, these dreams appear to be more mundane. We also know that dreams that occur during the NREM stage of sleep typically occur after midnight, which happens to be the time-period with the highest rate of REM sleep. This was discovered by a study whereby subjects took naps over specific periods of time and were then forcefully woken: sleep was divided into –

  • naps with only REM sleep, and
  • naps with only NREM sleep using polysomnography.

The implication here is that the occurrence of REM sleep is not necessary for dreaming, but rather that the actual processes creating REM sleep result in alterations to a person’s sleep experience.

By morning, and because of these changes, there occurs a sub-cortical activation that’s comparable to the one that occurs during REM sleep. Therefore, during the morning hours in the NREM stage it’s the sub-cortical activation that causes dreaming.

thumb stellate harmonie

Non-REM Muscle Movements

 The tonic drive to the majority of the upper airway’s respiratory muscles is repressed during non-REM sleep, with the following consequences –

  • Due to the intracellular calcium levels being lowered, weakened muscle contractions are caused by rhythmic innervation. Motoneurons are hyperpolarized by tonic innervation being removed.
  • There’s a ‘floppiness’ in the upper airway.

The diaphragm is typically driven by the autonomous system and is therefore normally spared non-REM inhibition, and as such, the suction pressure generated stays the same. The upper airway is thus narrowed during sleep, which increases resistance and makes upper airway airflow noisy and turbulent. As we know, we determine that a person is asleep by listening to their breathing: their breathing becomes distinctly louder once they fall asleep. So, it’s not surprising that the upper airway’s increased tendency to collapse while breathing during sleep leads to snoring, which is simply the tissues vibrating in the upper airway. For overweight people, this problem is aggravated when they sleep on their back because excess fat tissue can easily push down and close the airway. This can ultimately lead to sleep apnea.

Each sleep stage serves a unique role in brain and body restoration. There are many sleep deprivation studies that have demonstrated that depriving subjects of specific sleep stages of the sleep cycle has an adverse effect on body and brain functions.

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13 thoughts on “Stages of Sleep: The Sleep Cycle

  1. Ken Reply

    I didn’t see any mention of how long these sleep stages last. Without knowing that, I can’t relate my sleep problems to these cycles.

  2. John Reply

    I average less than an hour of deep sleep. Some times only about 20 min. The rest is 2x light sleep compared to rem sleep. Is this average?

  3. Mariana Ciureanu Reply

    I am a 75 years old woman with asthma. In 2019 I have been diagnosed with sleep apnea, even if I never had the somnolence which is typical for it and, according to my husband am snoring, but without the typical noises. On the contrary, I am a great insomniac: in order to sleep 6 hours per night I need to take Imovane ( 75mg) plus some naturopatic drugs containing hydroxytriptophan, melatonin, GABA and theanine. With the CPAP my insomnia has increased dramatically: after 5-6days of CPAP treatment I cannot sleep at all even with the drugs, and , in order to get to my normal, I need to take a pause of several days. I would like to know whether CPAP can have negative effects on asthma and whether it produces a decline in the REM part of the sleep. I would greatly appreciate your advise regarding my treatment with CPAP
    Dr.Mariana Ciureanu

    • Derek Reply

      Sleep technician here. I see many patients whose insomnia symptoms can become much worse if CPAP pressures are not correctly set. Ask your husband to watch you one night while wearing your CPAP to see if you have what is called Treatment Emergent Central Sleep Apnea. Central Sleep Apnea is basically when you stop trying to breathe entirely. If your CPAP pressure is too high, it may be causing you to stop breathing (not due to an obstruction though), and I see a lot of insomnia, specifically sleep-onset insomnia, to be related to cyclic central apneas when falling asleep. Fall asleep for 5 seconds, stop breathing, wake up for 5 seconds, fall asleep for 5 seconds, stop breathing, etc. Even 5 minutes of this can subjectively feel like you’ve been awake for hours. Ask your doctor to see if you can have an in-lab study with your current CPAP settings to see what’s happening; proper CPAP settings should never make your sleep feel worse. Hope this helps you and others!

  4. Saul Ramos Reply

    Why wake stage is not include I’m doing a sleep study research and I did not find wake stage here.

  5. saul Reply

    I’m doing research for sleep study and you did not include wake stage.

  6. Rebecca Reply

    What chemicals/hormones (?) are involved in allowing the body to go into deep sleep? For example to fall asleep GABA needs to rise, histamine and glutamate levels need to fall and that lets us feel sleepy. In the morning GABA levels reduce and histamine levels rise and we ‘wake’. What is involved in going into the deeps sleep stage??

  7. jaredh4 Reply

    Since you mention ‘delta’ sleep, you might as well mention the other synonyms for the other stages: alpha, beta, theta – after the wave forms seen in the EEG.

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