Jul 20, 20205 min read

Sleep

Updated: Oct 26, 2020

Sleep is an essential expenditure from the approximately 24-hour circadian rhythm. When awake, toxins tend to accumulate in the cranium due to the day's work. Studies claim that sleep provides an opportunity to clear these toxins from the Cerebrospinal fluid (CSF). By studying the alpha, beta, gamma and other brain waves, one can determine the state of any person's brain. There are three primary states of the brain.

Being awake (brain waves of higher frequency are seen)
Rapid Eye Movement (REM) sleep (a phase where a person sees most of his/her dreams. This phase is characterized by random and rapid eye movement along with alpha wave activity which is almost similar to patterns of an active brain)
Non-REM sleep (No eye movement) , which generally lasts for about 6-8 hours is a process of alternating REM and non-REM phases.

Human sleep involves both the hemispheres of the brain with both eyes closed. However, in some aquatic animals like dolphins, half of the brain sleeps at a time and the non-sleeping half (with the corresponding eye open), looks around for threats (unihemispheric slow-wave sleep).

The suprachiasmatic nucleus (SCN) is a tiny region in the Hypothalamus which takes care of the body's 24-hour rhythm (also called the circadian rhythm). These rhythms are feedback loops to external stimuli. Visual inputs are converted into electrical impulses in the eye with the help of photosensitive chemicals called opsins. These signals reach the SCN. If the surroundings are not dark, the SCN instructs the pineal gland to stop the production of melatonin (a hormone). In case it's dark outside, the SCN directs the Hypothalamus and pineal gland to produce cortisol and melatonin which contribute to making a person feel drowsy. Melatonin is an antioxidant that has been hypothesized to have a neuroprotective function. People who are visually impaired develop other methods to maintain their circadian rhythm. Externally supplying small doses of melatonin could also do the job. In organisms which hibernate, the circadian clock generally stops ticking.

Sleep is crucial to maintain a “normal” life. People today sleep much lesser than those who lived decades ago. Sleep deprivation has a significant impact on the functioning of our body. Melatonin receptors (structures which detect the hormone) are present around multiple tissues which are controlled by the central and peripheral nervous system. The pineal gland, which works in association with the SCN is the primary source of melatonin. Hence, sleep deprivation could affect melatonin production and in turn, affect the functioning of many organs.

Sleep deprivation could increase pain sensitivity and is also a major cause of motor accidents. People who are sleep deprived tend to lack empathy towards co-humans. It is also associated with anxiety and depression. Sleep deprivation is associated with reduced secretion of Leptin and Ghrelin (which control food intake and hunger) and therefore could lead to obesity. A neurotransmitter GABA (Gamma-aminobutyric acid) is also produced before sleep, which reduces activity in the Hypothalamus.

From the above figure, let’s focus on the cortex (which lies in the frontal lobe) and the hippocampus. The hippocampus plays an essential part in short term memory while the cortex that takes care of the long-term memory. The hippocampus – cortex interactions help us make and establish long term memories through the transfer of information from the hippocampus to the neocortex (which mainly happens when we’re asleep). It also helps in clearing up the hippocampus to facilitate the next day’s tasks. Random flashes of this information (from the hippocampus – cortex dialogue during sleep) manifest as dreams.

Dreams have been fascinating people for ages, and for some, they give a purpose to live life. A person could dream during both REM and non-REM sleep. Dreams tend to be more pleasant during the non-REM sleep and are "wild" during the REM sleep. To prevent any damage, the brain paralyzes the body during the REM sleep. During REM sleep, if the cortex switches from the dreaming mode to a state of being awake, a person would not be able to move due to paralysis (as per the brain's command). This condition is known as sleep paralysis and usually lasts for a few seconds.

So why did sleep and the process of dreaming evolve?

Sleep reduces body movements. Organisms could potentially reduce the risk of predation by reducing body movements. It also helps minimize resource competition. Suppose two animals are competing for food. The survival of both organisms would be at stake since resources are limited on earth. Instead, if one of them could evolve into a nocturnal being, competition could be reduced, and both animals could thrive (assuming that the other remains diurnal).

Organisms generally evolve to sleep when they are most vulnerable to predation. It helps them mimic the tonic immobility reflex (wherein an organism acts dead to escape predation). Dreams could be a visual rehearsal mechanism to cope up with predation pressure. Dreams could help humans develop their capacities by providing some visual worst-case scenarios. According to some researchers, modelling the different aspects like predation, food availability, environmental contribution, etc., sleep evolution could be inevitable.

Evolution of group behavior and societies have given our brains enough reasons to be depressed. Depression is a prolonged state of feeling low accompanied by emotional and physical exhaustion. Depression seems to have a link with sleep patterns and sleep itself! Depression results in poor sleep and extensive dreaming. People diagnosed with depression dream about three times more than others. This could be due to the emotional turmoil associated with depression. Greater the sleep deprivation, greater is the intensity of the REM sleep. This also means that sleep-deprived people take less time to reach the REM phase of sleep.

To reduce stress caused due to extensive dreaming, people dealing with depression are treated with antidepressants which work as a substitute for some sleep-inducing hormones. But this medication reduces the REM sleep duration, which makes the person feel tired and agitated when he/she wakes up.

In a nutshell, sleep is a product of the collective effort of both hormones and a tired brain. The following are some hormones which play a crucial role in influencing sleep patterns:

Melatonin – This hormone is produced by the pineal gland in association with the SCN. It controls the circadian rhythm. It helps a person in calming down and transcending into a state of unconsciousness.
GABA – It reduces activity in the brain (majorly in the hypothalamic region).
Adenosine – This chemical is a product of cellular consumption of energy. It supports sleep. Caffeine interferes with its receptor and effectively keeps us awake.
Cortisol – Cortisol is a stress hormone. Sleep deprivation causes an increased production of cortisol.
Adrenalin - A nightmare or a stressful dream causes increased sweating and heart rate. This response causes a person to wake up suddenly, and could also lead to difficulty in going back to sleep.
Cytokines – Chemicals like cytokines which are immunological response molecules, help us sleep.

Polysomnography which measures the brain waves, eye and leg movements, and metabolism are used to diagnose sleep disorders. Common sleep disorders include sleepwalking/talking, sleep apnea, insomnia, narcolepsy, disturbed circadian rhythms, etc.

With extensive research, sleep has graduated from being just a quiescent state to an important physiological process.

REFERENCES

A note about the author

Hema is a first year BS-MS student at IISER Kolkata who loves biology and wishes to purse a major in it. She also likes food, music and the coasts.

Sleep

So why did sleep and the process of dreaming evolve?

REFERENCES

A note about the author

Recent Posts

Comments

Subscribe Form