Most of you work nearly 9 – 12 hours a day (almost half a day) and year-round. Have you wondered how your mind and body allow you to do this and keep you going?
Indeed, only a healthy mind and physically fit body can do this. A restless mind or body can only keep you halted and unproductive. Sleep is the only function that keeps your day-to-day work shut, allowing a relaxing time for your mind and body. Sleeping keeps away the regular duties and initiates the brain and body rejuvenation process. Thus, when you wake up after a good night’s sleep, you’ll feel refreshed, energized, and with a well-working cognitive mind to think better.
Through the ages, people considered sleep as just another phenomenon or function. Only in 1953 that scientists discover that sleep comprises different stages (via electroencephalographic (EEG)) in which your body and brain do some restful parts to refurbish the whole system and get ready for the next day. Later, scientists also calculated that humans sleep about one-third of their total lifetime.
Post the initial finding in 1953, several pieces of research and studies began to understand sleep architecture, how it works, the stages, and their benefits to the body. Herein, let’s get to know what researchers have found.
Sleep Architecture
Sleep architecture indicates the basic structure, framework, and organization of normal or healthy sleep. The two types of sleep classification are non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Of note, the non-rapid eye movement sleep is divided further into stages: N1, N2, N3, and N4.
Each stage of sleep has its respective roles, functions, and benefits. In line with this, each stage varies from other stages in the aspects of eye movements, wave patterns in the brain signaling, working of muscles, organs, and hormones involved, and metabolisms.
Per night’s total sleep time, NREM sleep accounts for about 75% – 80%, and REM sleep accounts for the remaining 20% – 25%.
Sleep Cycle and Sleep Episodes
A sequence of various sleep stages or phases in episodes constitutes a sleep cycle. In humans, this cycle repeats every 1.5 – 2 hours during sleep. As the two types of sleep, REM and NREM sleep, follow the sequence, a sleep cycle is also called the REM-NREM cycle. However, scientists also term the sleep cycle as the ultradian or the sleep–dream cycle.
A sleep episode starts with an NREM Stage N1, proceeds with Stage N2, N3, N4, and ends with REM. Again, the onset of NREM Stage N1 occurs. This series of episodes iterate to make multiple sleep cycles in a single night’s sleep.
The first NREM-REM sleep cycle occurs between 70 – 100 minutes. The forthcoming cycles last longer than the first and occur between 90 to 120 minutes.
A typical sleep exhibits:
- A longer REM sleep when the night progresses and lasts the longest in the last sleep episode
- A longer NREM Stage N2 when the night goes
- The NREM Stages N3 and N4 shorten when the night progresses and sometimes fade away in the last sleep episode.
Stages of a Normal Sleep
NREM Stage N1 Sleep
In general, the sleep cycle begins with this stage of NREM sleep, and this phase acts as a transitional phase between wakefulness and sleep. It lasts just a couple of minutes in the range of 1 – 7 minutes, with 5 minutes being the average. Hence, it accounts for only 2% – 5% of the total sleep time.
As this phase transits between wakefulness and sleep, people can easily get distracted by any noise or sometimes sudden switching on of lights. With the onset of the transition;
- the brain activity changes from rhythmic alpha waves to low-voltage mixed-frequency waves
- the body temperature starts dropping
- the muscles change to relaxation mode
- the eyes start moving slower than before
Thus, it is called the light sleep stage, as light changes start occurring.
NREM Stage N2 Sleep
In this stage of NREM sleep, sleep spindles and K-complexes usually occur. Sleep spindles are nothing but powerful bursts that play a vital role in memory consolidation. People who learn new things tend to have high-density sleep spindles that work on memory aspects. Also, researchers found that these sleep spindles are responsible for disconnecting the brain from the environmental stimulus or any inputs irrelevant to sleep. In contrast, K-complexes are high voltage waves that respond to external noise, touch, or feel. These K-complexes wake you up with the onset of necessary stimuli if any.
Generally, this phase lasts about 10 – 25 minutes, and in the later cycles, this stage tends to last longer. Nearly half a night (45% – 55% of your sleep time) beholds this Stage N2 of NREM sleep.
- In this phase, the brain activity, the heart, and breathing rates slow down
- Unlike stage N1, slight noise cannot induce wake-ups. Instead, highly intense stimuli have to come into play
- The brain’s irregular electrical activities and mixed waves create sleep spindles
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NREM Stages N3 and N4
The NREM stages N3 and N4 together constitute Slow-Wave Sleep (SWS). Increased voltage pulses with slow-wave activity accompany the NREM Stages N3 and N4. The slow brain waves called the delta waves pave the way for getting into deep sleep mode.
The NREM Stage N3 lasts for about a couple of minutes, accounting for just 3% – 8% of the total sleep time. The NREM Stage N4 follows the NREM Stage N3 and lasts about 20 – 40 minutes. This accounts for nearly 10% – 15% of your sleep time.
Other characteristics of Slow-Wave Sleep include:
- Regular breathing
- Fall in blood pressure
- Pulse rate falls below the waking pulse rate
- Difficulty with quick wake-ups due to less receptive brain signals
REM Sleep
REM phase of sleep is a unique phase wherein rapid-eye movements occur. This stage also plays a vital role in memory consolidation. The characteristics of REM sleep include:
- Increased body temperature
- Desynchronized brain wave activity
- Sawtooth-like waveforms
- Theta and alpha activities
- The heart and breathing rate increases
- Blood pressure increases
- Prevalence of muscle weakness
In the initial sleep cycles, REM sleep lasts only about 1 – 5 minutes. But with the nocturnal progress and sleep episodes, the duration increases.
While at birth, REM sleep occurs for about 8 hours. The adult stage (20 years of age) reduces to nearly 2 hours. At an elderly stage (70 years of age), it even reduces to about 45 minutes.
Dreaming and REM Sleep
Dreaming occurs majorly in the REM sleep phase. Muscle tone and reflexes get lost to keep the body restricted or prohibit any externalization out of dreams. However, your brain works as in dreaming or thinking, and your eyes move back and forth behind the screens.
As dreaming occurs in the rapid-eye-movement (REM) sleep phase, scientists call this an active brain in a paralyzed body.
Physiological Differences between the NREM and REM Sleep
An article published by the Institute of Medicine (US) Committee on Sleep Medicine and Research tables the significant physiological differences between the NREM and REM sleep.
NREM Sleep
- The muscle tone is present and is similar to the wakefulness condition.
- Heart rate decreases than while in the awakened state.
- Blood flow slows down than while in the awakened state.
- Nerve activity decreases than while in the awakened state.
- Respiration decreases than the rate while in the awakened state.
- Body temperature reduces than while in the awakened state and is mostly regulated.
- Brain activity slows down than while in the awakened state.
REM Sleep
- The muscle tone is completely absent.
- Heart rate fluctuates (sometimes increases & sometimes decreases).
- Blood flow slows down than while at NREM sleep.
- Nerve activity significantly increases than while in the awakened state.
- Respiration usually increases along with brief stoppages.
- Body temperature is not regulated and varies with respect to that of the surrounding.
- Brain activity increases in a few specific areas (sensory elements).
Sleep Patterns at Different Stages of Life
In general, sleep efficiency reduces with age. From the infant to the adult stage, the way the sleep initiates, its quality, duration, and sleep efficiency undergo variations.
A Harvard health article says that slow-wave sleep attains its peak during young age, and with the increase in age, it declines.
A Healthline article presents a fact for healthy sleep:
- Newborns require around 14 - 17 hours of sleep a day
- Teenagers require about 8 - 10 hours of sleep a day
- Adults require about 7 - 9 hours of sleep a day
Body Functions and Benefits while at Sleep
Renal physiological changes that occur during sleep seem complex, involving changes in the renal blood flow, filtration, neural stimuli, and hormone secretion. In general, the renal changes result in concentrated urine flow due to decreased excretion of Na, K, Cl, and Ca in healthy individuals.
Until today, most of you would have thought that your brain does not work when you sleep. But, scientists, via experimental and observational studies, have proved that your brain plays the role of sorting information and creating memories.
The sympathetic nervous system that plays an essential role in stress responses and signaling during fight and flight relaxes during sleep.
The ADH hormone that accounts for not urinating while at sleep starts functioning with the onset of sleep. In addition to memory consolidation, the brain’s other function is to release this ADH hormone.
Hormones like Melatonin, Human Growth Hormone, and Thyroid Hormone get influenced by sleep. The melatonin hormone that enhances sleepiness secretes more during sleep. The human growth hormone that accounts for cell growth, reproduction, and regeneration gets secreted by the pituitary gland in the initial stages of the sleep cycle. Also, the thyroid hormone that regulates energy consumption and expenditure gets released with the onset of sleep (after evening).
Your body’s immune system activates the inflammation-fighting cytokines while at sleep. Hence, after an injury or infection, doctors usually recommend the patients to have a sound sleep to enable the innate immune system to work better at curing the damaged cells.
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Circadian Rhythm: The Sleep Regulator
Your body possesses a biological clock called the circadian rhythm that functions as a regulatory element of sleep. A group of cells in the brain’s hypothalamus constitutes the circadian rhythm. It works on a day-night cycle (24 hours) with the stimuli of illumination. Another group of cells called the suprachiasmatic nucleus (SCN) sends the signals from the retina to the hypothalamus to control the circadian rhythm. When the SCN senses the light, the circadian rhythm works in a day mode and vice versa.
The circadian rhythm plays a significant role in controlling the onset of sleep, different sleep stages, and the wake cycle every night throughout your life. In addition to managing sleep, the circadian rhythm also regulates food intake, body’s physical activity, hormone secretions, body temperature, and heart rate.
Factors affecting the Sleep Stages
A Harvard health article lists the two most important factors that researchers found influencing the various sleep stages.
Unwanted light during sleep affects the SCN signaling, affecting the circadian rhythm’s sleep regulatory function.
Any disturbance in the melatonin hormone (sleep hormone) secretion affects the individual’s sleepiness. Delaying the initiation of sleep affects the quality of sleep on a broader note.
Other external factors include caffeine intake, marijuana, lifestyle, sex, smoking, and sleeping pills.
Sleep Well! Live Well!