For much of human history, sleep has been a complete enigma. Only relatively recently have scientists began to uncover the reasons for why we, and all other animals on the planet, become unconscious and immobile for a third of our lives.
It might seem like not much is going on in our brains and bodies when we sleep, but in fact, both are bustling during this time for the ultimate benefit of our mental and physical health — and indeed, survival.
Let’s take a look at the fascinating science of sleep, and what happens in our brains and bodies when we drift off each night.
Sleep Cycles: The Periodicity of a Sleeping Brain
When we fall asleep each night, we enter into roughly ninety-minute sleep cycles that repeat four to five times until we wake up. These cycles are characterized by two distinct types of sleep, rapid eye movement (REM) sleep, and the multi-stage non-REM (NREM) sleep.
A full cycle involves the progression through the different stages of NREM sleep, to REM sleep, then starts all over again with NREM. NREM is longer than REM in the early cycles, but the imbalance in duration tilts back towards REM in the later cycles.
NREM sleep is divided into four stages of sleep, named stage 1 through 4. Stages 1 and 2 are the light NREM stages.
In the first stage of NREM, we are in a drowsy limbo between light sleep and awake. Eye movements begin to slow down, muscle tone reduces, body temperature drops, and brief hypnic jerks (muscle spasms) commonly occur in the body.
This relaxed state also corresponds to slower brain waves in the alpha and theta frequencies.
Awakenings can easily occur in this brief stage, lasting somewhere between five to ten minutes.
Stage 2 is officially light sleep. Here, our body temperature begins to drop and our heart rate and breathing slow down and become more regular. Slow eye movements cease, and our brain waves begin to slow further still. However, occasional bursts of activity spring forth at the tail end of each slow wave called sleep spindles.
Sleep spindles are thought to protect the brain from awakening from sleep and may play a role in memory consolidation and learning. We spend the majority of the night in Stage 2 NREM, which lasts about ten to twenty-five minutes, increasing with each successive cycle. This stage is important to consolidate learning and memory. It lasts about 20 minutes, making up about 20% of the total sleep cycle duration.
Stage 3 is the last stage of NREM and makes up roughly 15-20% of the total sleep period, but more in children and adolescents. It is referred to as slow-wave sleep, deep sleep, or sometimes delta sleep. It is characterized by synchronous, slow brain wave activity (called delta waves). These harmonious brain waves allow distant communication between brain regions and aid in the consolidation of long-term memories.
In stage 3, our blood pressure and heart rate further decrease and our breathing becomes deep, slow, and rhythmic. Our muscle tension also relaxes, but the muscles are still able to function. Therefore, parasomnias such as bed-wetting, sleep-walking, and night terrors can happen at this point.
It is hardest to awake in this stage. Loud noises, even up to 100dB in young adults (about the level of a chainsaw) can fail to arouse you. While it is the most difficult to wake up during this stage of sleep, if we do wake up, sleep inertia commonly results. This is characterized by feelings of grogginess and impaired performance that typically lasts fifteen to thirty minutes.
Slow-wave sleep is also the most restorative for the body. It is when the body releases hormones to repair and regrow tissues, builds bone and muscle, and strengthens immunity. Older people spend less time in deep sleep, and more time in stage 2 of NREM. To learn more about the importance of deep sleep (and how to get more of it), check out this article.
REM sleep is often referred to as “dream sleep” due to the presence of dreams during this time. When we enter REM sleep, the tone in our voluntary muscles completely disappears due to a signal originating from the deep, ancient part of our brain called the brain stem. This total paralysis state necessarily precedes the dreaming phase of this sleep period. When this is disrupted, it can result in what’s called REM sleep behavior disorder, where we physically “act out” our dreams.
When we do begin dreaming, there are phases in REM sleep where our eyes rapidly move back and forth. Why these precise movements happen is still an active research question, but some researchers believe the movements correspond to the processing of novel dream images. Our breathing and heart rate quickens and becomes more irregular. Our blood pressure also increases whereas our body temperature decreases to its lowest point.
In REM sleep, brain activity closely resembles what you see during wakefulness, oscillating at a relatively high frequency and in a chaotic manner. The decentralized, irregular activity stems from various regions of the brain that process memory, sensory stimuli, and emotion, which all work to produce the fantastical landscapes of dreams.
While NREM is associated with the storing and strengthening of relevant representations of the world, REM sleep allows us to integrate these past and present experiences into a coherent framework that can aid in problem-solving and personal insight.
One REM sleep cycle can last as short as ten minutes or as long as an hour, with longer durations increasing after each subsequent cycle.
The importance of sleep: How much do you really need?
How much deep sleep do we need?
In deep sleep, blood flow and blood glucose are regulated, our immune systems strengthen, our muscles rebuild, our tissues repair, new memories are consolidated, and our brain clears out metabolic waste that builds up when we are awake.
You might be wondering, given how restorative it is, how much deep sleep should I get? The answer depends on your age since sleep architecture changes substantially over a lifetime.
The recommended amount of sleep we need is seven to nine hours a night, and 13-23% of this is deep sleep in healthy adults. This comes out to roughly one to two hours of deep slow-wave sleep per night. Younger adults and adolescents will require the upper bound, approximately two or more hours of deep sleep per night.
Deep sleep decreases with age. Slow-wave sleep declines at a rate of about two percent per decade. Older people, due in part to changes in their circadian rhythm, generally report increased awakenings and reduced sleep efficiency compared to younger adults. They may be in deep sleep for as little as twenty minutes a night.
How much REM sleep do we need?
REM sleep is important for memory consolidation, learning and emotional processing, as well as brain development in infants. Early waking times can often short change us of this important stage of the sleep cycle.
When we do lose REM sleep, our brain tries to “steal” it back through a process called REM rebound, where we spend more time in REM sleep the next night. On the other hand, excessive REM sleep is associated with affective disorders like depression.
In healthy adults, about 20-25% of the total sleep duration is spent in REM, with most of it taking place in the second half of the night. For eight hours of sleep, that is about one and a half to two hours. In babies and children, nearly half of their total sleep time is spent in REM, which reflects its importance for sculpting the brain’s connections and regulating mood, memory, and emotions.
It’s clear that our brains and bodies are anything but dormant when we fall asleep each night, and so a good night’s sleep is fundamentally important. In fact, in REM sleep, our brain activity is sometimes higher than when we are awake.
If you think you are consistently not getting enough sleep, consider consulting with a sleep medicine professional. A sleep study can be performed that records brain and body activity to track your sleep cycles and rule out any underlying medical cause.
Much of the science of sleep is still terra incognita, but we are increasingly understanding its vital importance to virtually every aspect of healthy living. Even though sleep is sometimes called the “cousin of death”, it is truly crucial to life.
Slow wave: Rhythmic patterns of brain activity with a frequency of less than 4 Hz (cycles per second). Associated with stage 3 and stage 4 of NREM, otherwise known as “slow-wave sleep” (SWS)
Sleep Spindles: Bursts of neural activity with a frequency of 11-16Hz during stage 2 NREM sleep. Thought to play a role in memory consolidation and shielding the brain from awakening from external sensory stimuli
Brain wave: Rhythmic patterns of neural activity in the brain. It can be recorded by electrodes placed on the scalp, which records the electrical activity of large groups of neurons.
Alpha wave: Neural oscillations in the 8-12Hz (cycles/second) frequency range. These brain waves are associated with a relaxed, alert mental space as seen in stage 1 of NREM and meditation.
Theta wave: Neural oscillations in the 3-8Hz frequency range. These brain waves are found in the transition to light sleep, REM sleep, and deep meditation.
Delta wave: Neural oscillations in the 0.5Hz-4Hz range. They are associated with stages 3 and 4 of NREM (slow-wave sleep).
Memory consolidation: Process by which a temporary, fragile memory is restructured and transferred into a stable, long-lasting memory.
Parasomnia: Sleep disorders that involve abnormal physical or verbal behaviors that disrupt sleep. May occur in arousal from REM sleep or NREM sleep.
Sleep Inertia: A temporary grogginess and sensorimotor impairment that occurs after awakening. It can last from 30 minutes to two hours.
Brain stem: One of the evolutionarily oldest parts of the brain that connects the brain to the spinal cord. Controls basic survival mechanisms such as heart rate, breathing, sleeping, and digestion.
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