There are many different ways that the human brain may work, some of which are being researched now. The sleep cycle is a process that we have researched extensively and for which we have produced a methodical explanation. The mind’s capacity to shift from consciousness—a level of awareness that allows us to make sense of everything around us at all times—to unconsciousness, which occurs every night as the body cycles through various phases of sleep, is vital to human life. Sleep is a means of mental and physical renewal. A person will cycle through the five distinct phases of sleep that have been identified thus far throughout a complete night’s sleep. Although seven hours is generally considered a “good night’s sleep,” everybody needs a different amount of sleep to operate at their best.
Stages of Sleep Cycle
Bio-Function of Sleep
The circadian rhythm, which is fueled by the SCN, controls the sleep cycle. Adrenocorticotropic hormone (ACTH), prolactin, melatonin, and norepinephrine (NE) are among the other chemicals whose nocturnal release is regulated by the circadian rhythm.
The SCN signals the brain to produce more melatonin when there is less light, such as at night, so that you can feel sleepy and eventually fall asleep. Melatonin levels will decrease after a night’s sleep, causing you to wake up during the day.
Mechanism
REM sleep and non-REM (or non-rapid-eye movement) sleep are the two main categories of sleep. Darting eye movements beneath closed eyelids are a hallmark of rapid eye movement (REM) sleep. Brain waves during REM sleep and wakefulness appear to be extremely similar. On the other hand, distinct brain wave patterns separate the three phases of non-REM sleep from each other and from alertness. REM sleep is the fourth and last stage of sleep, whereas NREM sleep comprises the first three stages. This section will go over each of these sleep phases and the brain wave activity patterns that correspond with them.
Non-REM Sleep
The physiological activity of the body decreases during non-REM sleep as biological processes slow down. Non-REM sleep occurs in three stages, which are known as N1, N2, and N3.
Every stage has distinct features that set it apart from the others, such as the degree of sensory and motor disconnections or the depth of sleep. Non-REM sleep is characterized by a reduction in heart rate, breathing, muscular activity, growth hormone release, brain electrical activity, and oxygen consumption (Purves et al., 2001).
Numerous brain regions, particularly those of the thalamus and cerebral cortex, govern non-REM sleep (De Andrés, Garzón, & Reinoso-Suárez, 2011).
REM Sleep
Contrarily, REM sleep is characterized by very active brain activity and is a far more active stage of sleep than non-REM. The brainstem, the area of the brain that joins the cerebrum and spinal cord, has a major regulatory role during this period (McCarley et al., 1995). It is made up of the pons, medulla oblongata, and midbrain. Every ninety minutes or so, the brain goes through stages one, two, and three before entering REM sleep (McCarley et al., 1995).
Rapid eye movements, dreams, increased brain activity, and inhibition of voluntary muscles all take place during REM sleep (McCarley et al., 1995).
Sleep Stages
William Dement and Nathaniel Kleitman published a groundbreaking work in 1957 that described the individual stages of sleep that collectively comprise the internal cycle that takes place each night while we sleep (Dement & Kleitman, 1957).
Stage 1 (N1): Between awake and sleep, or the time when we go off to sleep, is a transitional state known as sleep. Both the pulse and respiratory rates drop during this period. Additionally, there is a noticeable drop in core body temperature and total muscular tension during stage 1 sleep.
Stage 2 (N2): This stage is characterized by features similar to N1, including a further slowing of respiration and pulse and even more muscular relaxation than in N1 (Lockett, 2020). Stage 2 (N2) is still a time of light sleep. Theta waves are the hallmark of stage 2 (Abeln et al., 2014), but the brain also starts to generate sleep spindles—bursts of fast, rhythmic brain wave activity—which are crucial for memory and learning (Fogel & Smith, 2011). K-complexes, high amplitude patterns of brain activity that may arise in response to environmental cues, are present in addition to sleep spindles (Forget et al., 2011). N2 lasts for around twenty minutes in total (Altevogt & Colten, 2006).
Stage 3 (N3): These stages of sleep are marked by low frequency (up to 4 Hz), large amplitude delta waves, and are sometimes referred to as deep sleep or slow-wave sleep (Figure 4). An individual’s breathing and heart rate drastically decrease during this period. Compared to earlier stages, stage 3 sleep is far harder to rouse someone from. Interestingly, regardless of how long they slept, people who have higher levels of alpha brain wave activity during stage 3—which is more frequently linked to wakefulness and the transition into stage 1 sleep—often report that they do not feel refreshed when they wake up (Stone, Taylor, McCrae, Kalsekar, & Lichstein, 2008).
REM Sleep & Dreams
As noted, fast eye movements are a sign of REM sleep. Dreaming happens during this stage of sleep, which is characterized by brain waves that are quite similar to those seen while a person is awake. It is also linked to paralysis of the body’s muscular systems, excluding those necessary for breathing and circulation. Because of this mix of intense brain activity and absence of muscular tone, REM sleep is sometimes referred to as paradoxical sleep—in healthy individuals, no voluntary muscle movement occurs during this stage of sleep. Similar to NREM sleep, REM sleep has been linked to certain facets of learning and memory (Wagner, Gais, & Born, 2001). However, scientists cannot agree on the significance of either NREM or REM sleep for typical learning and memory (Siegel, 2001).
Dreaming is one of the main features of REM sleep. According to Tirapu-Ustarroz (2012), dreams are a series of thoughts, feelings, sensations, and pictures that often happen unconsciously in the mind.
Studies showing that people woken from REM sleep recall complex, vivid, emotional dreams, while subjects woken during non-REM sleep report fewer, less vivid dreams, provide evidence that dreams happen during REM sleep (Purves et al., 2001).
Though theories suggest that we may dream to help further develop cognitive abilities (Medrano-Martinez & Ramos-Platón, 2005), reflect unconscious mental functions in a psychoanalytic way (Eiser, 2005), prepare for potential threats (Valli et al., 2005), or consolidate memories (Pace-Schott, 2013), the meaning of dreams and why we dream are still largely unknown.
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