Gina Johnson
Sleep is a complex and dynamic process that affects how we function in ways scientists are only beginning to understand. Sleep accounts for one-quarter to one-third of our lives, and during this time, our brains cycle through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep. Each sleep cycle is made up of four stages, with the first three being non-REM sleep and the fourth being REM sleep. During non-REM sleep, our heart rate and breathing regulate, and our body temperature drops. In contrast, during REM sleep, our eyes move rapidly, our breathing becomes faster and irregular, and our heart rate and blood pressure increase. While we sleep, our brain and body repair, restore, and re-energize, and our brain stores new information and gets rid of toxic waste.
| Characteristics | Values |
|---|---|
| Sleep cycles | Each sleep cycle takes between 70 and 120 minutes and consists of four stages, three of which are non-REM sleep stages |
| Non-REM sleep | First stage between being awake and falling asleep; second is light sleep; third and fourth are deep sleep |
| REM sleep | Occurs 90 minutes after falling asleep; eyes move rapidly; brain waves are similar to those during wakefulness; breath rate increases; body becomes temporarily paralyzed |
| Sleep duration | 7-9 hours for adults; more for children and teens |
| Sleep deprivation | Insomnia; sleep deprivation |
| Hormone production | Increased levels of growth hormone; decreased levels of cortisol |
| Body temperature | Decreases as bedtime approaches |
| Heart rate | Decreases during non-REM sleep; increases during REM sleep |
| Brain activity | Brain cells settle down during non-REM sleep; during REM sleep, brain activity is similar to when awake |
| Memory | Memory consolidation likely requires both non-REM and REM sleep |
| Biological mechanisms | Circadian rhythm and homeostasis regulate sleep-wake cycles |
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What You'll Learn
Sleep cycles: non-REM and REM sleep
Sleep is a complex and dynamic process that affects how we function in ways that scientists are only beginning to understand. During a normal sleep period, humans progress through four to five sleep cycles, each made up of four individual sleep stages.
The four stages of sleep are further divided into two categories: non-rapid eye movement (non-REM) sleep and rapid eye movement (REM) sleep. These categories are important because what happens during REM sleep is dramatically different from what happens during non-REM stages. The first three stages of sleep are composed of non-REM activity.
Non-REM sleep is composed of four stages. The first stage comes between being awake and falling asleep. This stage usually lasts only a few minutes, making up about 5% of your sleep time. The second stage is light sleep, when heart rate and breathing regulate and body temperature drops. This stage can last for 10-25 minutes. The third and fourth stages are deep sleep. It is harder to wake someone up during this stage, and if they do wake up, they will probably experience sleep inertia, a state of confusion or mental fog that lasts about 30 minutes.
After the three non-REM stages, the body moves into the REM stage. During this stage, the eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. Breath rate increases, the heart rate and breathing quicken, and the body becomes temporarily paralyzed as we dream. The cycle then repeats itself, but with each cycle, less time is spent in the deeper stages three and four of sleep and more time in REM sleep.
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Sleep architecture: the progression of sleep stages
Sleep architecture refers to the progression of sleep stages and cycles in one sleep period. A typical night's sleep consists of four to five sleep cycles, with each cycle made up of four stages. These four stages can be further broken down into two categories: rapid-eye movement (REM) sleep and non-REM sleep.
The first stage of sleep is non-REM sleep, which is composed of three stages. The first stage, N1, occurs when a person first falls asleep and usually lasts one to seven minutes. During this stage, the body hasn't fully relaxed, but body and brain activities start to slow, with periods of brief movements. It is easy to wake someone up during this stage, but if they are not disturbed, they will quickly move into the second stage.
The second stage, N2, is when the body starts to relax more deeply. The body temperature drops, muscles relax, and the heart and breathing rate slow. Brain waves slow down and have noticeable pauses between short, powerful bursts of electrical activity. Experts believe that these bursts are the brain organizing memories and information from when the person was awake. Stage 2 NREM sleep accounts for about 45% of total sleep time and is the stage where most people spend the majority of their time.
The third and fourth stages of non-REM sleep are deep sleep. During these stages, the brain produces slow but strong brain waves. The body takes advantage of this very deep sleep to repair injuries and reinforce the immune system.
After progressing through the three stages of non-REM sleep, the body then moves into REM sleep. During this stage, the eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. Breathing becomes faster and irregular, and the heart rate and blood pressure increase to near-waking levels. The body's arm and leg muscles become temporarily paralyzed, preventing the sleeper from acting out their dreams. Most dreaming occurs during REM sleep, although some can also occur during non-REM sleep.
After the first cycle of non-REM and REM sleep, the body starts a new sleep cycle and returns to stage 1 or 2 of non-REM sleep, and the cycle begins again. Each cycle normally takes about 90 to 120 minutes. As the night progresses, the body spends less time in the deeper stages of non-REM sleep and more time in REM sleep.
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Circadian rhythms: controlled by a biological clock
Circadian rhythms are 24-hour cycles that are controlled by a biological clock located in the brain. One of the key functions of this biological clock is to respond to light cues, increasing the production of the hormone melatonin at night and switching it off when it senses light. This is why people with total blindness often have trouble sleeping.
The circadian rhythm directs a wide variety of functions, from daily changes in wakefulness to body temperature, metabolism, and the release of hormones. Circadian rhythms are closely linked to the sleep-wake cycle, and disruptions to this rhythm can cause sleeping problems. For example, night shift workers often have trouble falling asleep when they go to bed and staying awake at work because their natural circadian rhythm is disrupted. Jet lag also interferes with a person's circadian rhythm, creating a mismatch between their internal clock and the actual clock.
The suprachiasmatic nucleus (SCN) is a cluster of thousands of cells within the hypothalamus that receives information about light exposure directly from the eyes and controls behavioural rhythms. People with damage to the SCN sleep erratically throughout the day because they cannot match their sleep/wake cycle with the light-dark cycle.
The circadian rhythm works together with homeostasis to regulate when an individual is awake and when they are asleep.
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Sleep and hormones: the body makes more of some hormones
Sleep is a complex and dynamic process that affects how we function in ways that scientists are only beginning to understand. Sleep accounts for one-quarter to one-third of the human lifespan. During sleep, the brain cycles repeatedly through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep. Each sleep cycle takes between 70 and 120 minutes, and in the first sleep cycles of the night, more time is spent in non-REM sleep.
During non-REM sleep, the body enters a restorative state, and the brain produces more thyroid hormones and growth hormones. Levels of cortisol, sometimes called the "stress hormone," decrease when a person first falls asleep. Non-REM sleep is also when the body repairs cells, restores energy, and releases molecules like hormones and proteins.
During REM sleep, levels of the "stress hormone" cortisol increase, and the body prepares to wake up. REM sleep is when most dreaming occurs, and the eyes move rapidly from side to side behind closed eyelids. The brain is highly active during REM sleep, with brain waves similar to those during wakefulness.
The balance of hormones such as leptin and ghrelin, which control hunger, can be disrupted by a lack of sleep, leading to changes in eating habits and weight gain. Sleep also affects the production of melatonin, which is controlled by our biological clock's response to light cues. People with total blindness often have trouble sleeping because they cannot detect these light cues.
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Sleep and brain function: the brain stores new information
Sleep is a complex and dynamic process that is necessary for human beings. It is regulated by two main processes: circadian rhythms and sleep drive. Circadian rhythms are controlled by a biological clock located in the brain, which responds to light cues and regulates the production of melatonin. Sleep drive, on the other hand, is influenced by factors such as medical conditions, medications, stress, sleep environment, age, and diet.
During sleep, the brain cycles through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep. Each sleep cycle consists of four stages, with the first three stages being non-REM sleep and the fourth being REM sleep. While non-REM sleep is characterized by slower brain waves, lower heart rate, and regular breathing patterns, REM sleep exhibits brain activity similar to that of a waking state, with faster and more irregular breathing, and increased heart rate and blood pressure.
The brain plays a crucial role in sleep regulation and function. The hypothalamus, a small structure within the brain, contains the suprachiasmatic nucleus (SCN), which is responsible for receiving information about light exposure and controlling behavioral rhythms. Additionally, the brain stores new information and gets rid of unnecessary data during sleep. This process of memory consolidation likely requires both non-REM and REM sleep.
During non-REM sleep, nerve cells in the brain slow down and settle into a steady, rhythmic pattern. This stage is crucial for learning and memory retention, providing a restorative and restful state for the brain and body. As we progress into REM sleep, brain activity increases, resembling wakefulness. Dreaming occurs during this stage, and brain cells fire actively and randomly.
In summary, sleep is a period of intense brain activity, with the brain cycling through different stages of sleep. The brain stores new information, consolidates memories, and supports overall brain health and function. The interplay between non-REM and REM sleep stages facilitates these processes, highlighting the importance of obtaining sufficient and quality sleep for optimal brain function.
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Frequently asked questions
Humans cycle through four stages of sleep: non-REM sleep, which is composed of four substages, and REM sleep. Non-REM sleep is a period of deep sleep where the body repairs and restores itself, while REM sleep is a more active period of sleep where brain activity is similar to that of wakefulness, and dreaming occurs.
During non-REM sleep, the body enters a state of relaxation. Heart rate and breathing slow down, body temperature drops, and blood pressure decreases. The brain produces slower, more rhythmic brain waves.
During REM sleep, the eyes move rapidly behind closed eyelids, and brain waves are similar to those during wakefulness. Breathing becomes faster and more irregular, and heart rate and blood pressure increase to near-waking levels. Dreaming occurs, and the body becomes temporarily paralyzed.
Humans regulate their sleep-wake cycles through a combination of circadian rhythms and sleep drive. Circadian rhythms are controlled by a biological clock located in the brain, which responds to light cues and regulates the production of the hormone melatonin. Sleep drive refers to the body's need for sleep, which builds up the longer a person stays awake.
When humans don't get enough sleep, they may experience negative effects on their thinking, concentration, energy levels, and mood. Sleep deprivation can disrupt the balance of sleep architecture, leading to daytime sleepiness and impaired cognitive function.
