Understanding the Brain’s Rhythms: Sleep, Wakefulness, and Beyond

Human behavior, like all mammals, can be divided into two periods: sleep and wakefulness. The normal course of these phases is determined by the functioning of the body systems. Depending on what goal we want to achieve, different centers of the brain are activated to switch between functional states.

We can feel such shifts subjectively: for example, when we focus and delve into the text while reading superficially. At this moment, our functional state changes from resting to more attentive. This change can be seen on the encephalogram (EEG). The electrochemical activity of the brain is expressed in oscillations of different frequencies, or in rhythms. In total, five main rhythms are distinguished, which differ in frequency range:

1. delta rhythm a frequency of 0-4 Hz can be called the rhythm of complete rest and serenity, it is characteristic of the phase of deep sleep.
2. Theta rhythm a frequency of 4–7 Hz is also fixed in a state of calm, during drowsiness, during the transition from wakefulness to sleep, and also when referring to old memories.
3. alpha rhythm – the most common, its frequency is 8-14 Hz. It manifests itself during relaxation and rest. For example, in the morning on a weekend, when you don’t have to go anywhere; when we are calmly awake, when we do not focus attention and do not solve any problems. If you have ever had an EEG done, you should have been asked to close your eyes at the beginning of the procedure, because the alpha rhythm is most pronounced when the eyes are closed.
4. beta rhythm a frequency of 14–30 Hz is recorded during concentration of attention, active wakefulness, solving everyday, but not particularly complex tasks. May be stimulated by caffeine, nicotine and mental activity.
5. Gamma rhythm a frequency of 30–100 Hz appears during active learning, the creative process, and solving complex problems. This is the state of “flow”, when we are purposefully engaged in certain activities and are not distracted by extraneous matters. It is also most pronounced when using short-term memory and working with large amounts of information.

Depending on our activities, the basic rhythm of the brain can change. For example, right now, while reading this article, you are most likely in a relaxed state – the ensembles of neurons in your brain generate an alpha rhythm. During work or study, the beta rhythm will prevail. What is the state of the brain during sleep? The study of this began with the EEG of a sleeping person, which did H. Berger in 1928.

Sleep can be defined as a special state of warm-blooded animals, which is characterized by a successive change of phases and is a genetically predetermined program. Its main function is considered to be the rest of the body.

There are two main parts to sleep – non-REM sleep and REM sleep. The first begins immediately after a person has fallen asleep, and his brain is at rest. The second comes after the slow phase and is characterized by increased brain activity and rapid eye movement (Rapid Eye Movement), which is why it is called REM sleep, or paradoxical sleep. The paradox is that, despite the surge in brain activity, the muscles of the body remain completely relaxed.

Night sleep is cyclical, with each cycle consisting of five consecutive phases with a total duration of one and a half hours. In total, there can be 4–6 such cycles per night. The change in EEG rhythms serves as an objective indicator of the change in sleep phases. The first four are non-REM sleep, in which slow-wave oscillations predominate. The latter is REM sleep.

1. The first phase – nap – serves as the transition of the body from the state of wakefulness to sleep. There is a decrease in alpha waves and the spread of theta waves. In this phase, half-asleep dreams appear, the body relaxes, the body prepares to go to sleep.
2. In the second phase, “sleep spindles” appear on the EEG – alpha waves of 12–14 Hz, which indicate a disconnection of consciousness. The threshold of perception is increased, it is easy to wake a person, hearing is especially sensitive: even a slight noise can wake a sleeping person.
3. The third phase is preparatory before moving on to deep sleep.. Delta waves of high amplitude begin to dominate, which mean that a person falls into a state of sleep.
4. Fourth phase – deep sleep. Delta waves of 2 Hz predominate. It is difficult to wake a person up, dreams are possible that are not remembered after waking up.
5. Fifth phase – REM sleep. Brain activity on the EEG is almost the same as during wakefulness. However, wakefulness and REM sleep are provided by different areas of the brain. As the discoverer of REM sleep, Michel Jouvet, said, this is the third state of the body, which is equally different from both slow-wave sleep and wakefulness.

The widespread opinion that the body rests in a dream is not entirely true: it continues to work, only in a different mode. Scientists still disagree about the main function of sleep – there are many different theories on this subject. Apparently, it is primarily needed to restore the levels of hormones and neurotransmitters required to maintain wakefulness.

Sleep is a fairly new invention of evolution, it appears only in warm-blooded animals. In cold-blooded animals, there is a decrease in metabolism and body activity, but this state cannot be called a full sleep.

Before understanding how the body provides the state of sleep, it is necessary to talk about wakefulness systems. The most important conductor of our activity is the so-called reticular formationopened in the late 1940s. It is a mesh structure that extends throughout the brainstem. The brain stem is a stem-like structure that connects the brain to the spinal cord. It includes the midbrain, medulla oblongata and pons.

The main function of the reticular formation is to maintain the tone of the body. The neurotransmitter for this is glutamate, without which the brain simply falls asleep and stops responding to external stimuli. There are other wakefulness centers scattered throughout the brain:

• pedunculopontine nucleuswhich releases acetylcholine and is involved in the regulation of motor control and cognitive processes;
• seam core – one of the main sources of serotonin, responsible for calmness and high spirits, as well as for the regulation of sleep-wake cycles;
• tubero-mamillary nuclei are responsible for histamine, the “waking neurohormone”. You’ve likely heard that a side effect of antihistamines for allergies is drowsiness. This is due to the blocking of histamine. It rises upon awakening and decreases before going to bed;
• hypothalamus highlights orexin and melanin-concentrating hormone (MSH). Orexin is a protein neurotransmitter that has been found in all mammals. It is synthesized by a group of cells of the posterior and lateral hypothalamus, whose axons branch, reaching almost all parts of the brain. The main function of the orexin system is to maintain wakefulness and trigger search-orientation behavior, which is expressed in the subjective feeling of interest and curiosity. It also makes the transition from wakefulness to sleep smooth and consistent. Narcolepsy is explained precisely by disturbances in the work of the orexin system;
• suprachiasmatic nucleus – the biological clock of our body and circadian rhythm generator. Its processes are closely connected with the pineal gland, which secretes the sleep hormone melatonin;
• basal nuclei, especially the Meinert kernel, which helps us remember information and concentrate. It also triggers beta and gamma rhythms of wakefulness with increased brain work;
• and other areas of the brain.

If there are so many centers of wakefulness, it is logical to assume that there is at least one center of sleep. His searches have been conducted since the time of the physiologist I.P. Pavlov, but without success. It turned out that the centers of wakefulness contain numerous feedback mechanisms responsible for sleep. The only contender for a kind of “sleep center” – ventrolateral preoptic nucleus in the anterior hypothalamus.

Change of sleep phases

This nucleus inhibits wakefulness systems with the help of the neurotransmitters GABA and galanin. GABA promotes the transition from wakefulness to slow-wave sleep. Galanin supports and enhances the inhibitory effects. In galanin-impaired rats, non-REM sleep is initiated but not sustained for long.

An objective indicator of sleep is slow-wave brain rhythms on the EEG. For the behavior of waves during slow sleep, the fence of the brain is primarily responsible. Scientists discoveredthat the dendrites of some of the neurons in this nucleus basalis extend almost throughout the entire brain. With their help, the fence synchronizes slow waves in different areas of the brain.

REM sleep is triggered from a well-defined area near the pons and medulla oblongata. The EEG pattern during wakefulness and REM sleep is almost identical, but the difference lies in the fact that in the first case, monoamines (norepinephrine, serotonin and histamine) predominate in the brain, and in the second – acetylcholine and glutamate.

This provides the paradoxical nature of such a state – high activity of the brain during external rest.

The phase change during sleep occurs in the pontine tegmentum, which also maintains the body’s homeostasis. According to models neuroscientist Alan Rosenwasser, two types of neurons are involved in the shift:

• REM-on-neurons trigger REM sleep by activating the nucleus, which releases acetylcholine.
• REM-off-neurons slow down the “switches” of REM sleep, activating the blue spot (noradrenaline), the dorsal raphe nucleus (serotonin), the lateral hypothalamus (orexin), which puts us into a slow phase of sleep.

Depending on which neurons are active, one or another phase of sleep is turned on. Today, REM sleep is believed to be necessary for the proper functioning of genetic programs, gene expression, the formation of new skills and memory retention, as well as to eliminate unwanted neural connections (that is, to “unlearn” a person from something).

Causes of Narcolepsy

One of the most famous sleep disorders is narcolepsy, first described by the French scientist Edouard Gelino in the 1880s. Its main symptoms are:

• sleepiness during the daybecause narcoleptics do not get adequate sleep at night;
• cataplexy – complete and sudden relaxation of the muscles, when a person can neither move nor speak;
• hallucinations when falling asleep and waking up;
• sleep paralysis, coming due to the peculiar influx of the phase of REM sleep on awakening, is accompanied by the remnants of dreams. In this state, wakefulness and dreaming are mixed, which provokes unpleasant, sometimes frightening impressions.

The severity of symptoms can be different: some patients retain contact with reality, others completely lose. Although this is not a fatal disease, it greatly disrupts a person’s normal life.

The main cause of narcolepsy is a disturbance in the orexin system, which provides a smooth transition between the slow phase and wakefulness. It stops signaling the monoamine system, in particular the blue spot that produces norepinephrine. Due to the lack of monoamines, narcoleptics begin to produce acetylcholine in large quantities, which provokes the onset of REM sleep. An attack can be stopped by the introduction of orexin.

Research showedthat it is an autoimmune disease that is only transmitted if both parents are narcoleptics. In recent decades, scientists have made significant progress in identifying genes narcolepsy. The death of orexin neurons is associated with leukocyte antigens, which play an important role in the regulation of the immune system. One of these genes increases the susceptibility of orexin neurons to immune attack, which leads to their death.

Most people with narcolepsy have this gene, but there are other factors as well. There is still no satisfactory therapy for narcolepsy. Doctors prescribe antidepressants to increase the activity of the monoamine system, but in parallel, a drug is being developed that would stimulate the production of orexin.