Neuro Science |
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What is Sleep ?
I think all the readers would sleep. Then can you explain what is sleep ? Oh..O. Not easy !! There may not be a single / clear cut definition of sleep even when you ask experts. In this kind of situation, a typical approach is to try to figure out what exactly happens when we sleep. This also may not be easy but everybody would be able to come up with at least a few observations / experiences.. something like below. Just try to speak out whatever comes up in your mind regardless of whether it is right or wrong
In this note, I will talk about various aspect of the activities during sleep and followings are the list of topics to be covered.
Does every animal sleep ? Scientists believe all animal sleep. but the duration and patterns of sleep can vary greatly among different species. Some animals, such as dolphins and certain birds, are able to enter a state of rest called unihemispheric slow-wave sleep, in which only one half of their brain sleeps at a time, allowing them to continue to swim or fly and remain alert for potential predators. Other animals, such as bats, can enter into a state of torpor, which is a form of hibernation-like sleep that allows them to conserve energy during periods of food scarcity.
We would know that higher animals like mammals would sleep just by direct observation. But there can be various cases where it is hard to know whether a specific animals sleep or not. However it is known by scientists that even very simple animal like Hydra sleeps.
Sleep is so essential that some animals sleeps only in a part of the brain when they cannot sleep with both brains. For example, a seal while floating on the sea it cannot go to sleep with the whole brain since it will drown to death if they sleep with the whole brain. In this csae, they sleep in only half of the brain
How do we know if they sleep ?
How do you know if an animal sleep ? If the animal is a human, it is relatively easy to figure out whether they are sleep or not. You would know how to know for human without any further explanation excep for some rare case for those who sleep without closing eyes or those sleeping sitting in chair without closing eyes etc :).
Followings are some of the most common indicator to checkif an animal is in sleep or not.
If you are a researcher in this area, the best way would to be use various measurement equipment like EEG(ElectroEncelphalGram), EMG (ElectroMyoGram), EOG(Electro-oculography machine) etc. You may use these method for most of higher animals like mammal, birds and even for some reptiles (like crocodiles).
How about for lower animals like Fish, Hydra etc ? In those case, researchers mostly use the activity (e.g staying still without moving) or responses to stimulus as indicators for sleep.
Researchers have observed that even single-celled organisms, such as some species of protozoa, display patterns of activity and inactivity that are consistent with a sleep-like state. In more complex animals, scientists have used a variety of methods to study the neural and physiological changes that occur during sleep, such as monitoring brain activity, measuring changes in hormone levels, and observing changes in muscle tone and movements. Additionally, studies have also been conducted to see the consequences of deprivation of sleep, which have shown that sleep deprivation can have negative effects on the behavior, cognitive abilities, and overall health of animals.
Why all animal sleep ? (at least, why all animal seems to sleep ?). In short, we don't have any clear answer that fit for all animals. We are just making guesses from various researches and making a list of possible reasons for sleep. Some of the most commonly mentioned reason for sleep are as follows.
Following is an example on how sleep impact on Memory Process. As shown in [a], you would notice :
Image Source : Declarative memory consolidation in humans: A prospective functional magnetic resonance imaging study
Impact on Cleaning up the metabolic waste
In following research, radio labelled chemicals (Amyloid Beta, Inuline) are injected in prefrontal cortex of mice in awake, sleep and anesthetized state and measured how fast those chemicals cleared out of the brain. Brains were harvested 10 to 240 min later for analysis.
KX : anesthetized with ketamine/xylazine Image Source : Sleep Drives Metabolite Clearance from the Adult Brain
[A],[C] shows the recovery late over time. Lower value indicates that more of the materials has been washed out (cleared) [B],[D] shows the rate of the clearance. Higher value indicates that the materila is washed our more quickly.
The result shows that both Amyloid Beta and Inuline gets washed out (cleared) more rapidly during sleep or anesthetized than awake.
NOTE : Amyloid beta
Amyloid beta is a peptide that is involved in the development of Alzheimer's disease, a neurodegenerative disorder characterized by memory loss and cognitive decline. Amyloid beta is a protein fragment that is produced by the abnormal cleavage of a larger protein called amyloid precursor protein (APP). In Alzheimer's disease, amyloid beta peptides tend to aggregate and form plaques in the brain, which are thought to contribute to the death of nerve cells and the development of the disease.
Research has shown that brain energy consumption during sleep is generally lower than during wakefulness.
One study using positron emission tomography (PET) imaging found that during NREM sleep, brain glucose metabolism (a measure of brain energy consumption) is about 6-8% lower than during wakefulness. During REM sleep, brain glucose metabolism is about 10-20% lower than during wakefulness. Additionally, a study using functional magnetic resonance imaging (fMRI) found that during NREM sleep, brain activity in the prefrontal cortex, which is involved in higher cognitive functions such as attention and decision making, is significantly reduced compared to wakefulness.
Following figure shows energy consumption changes in brain during wake and sleep stage
Image Source : Brain Energetics During the Sleep-Wake Cycle
Following figure shows energy consumption changes in overall body part during wake and sleep stage
Image Source : Effect of shortened sleep on energy expenditure, core body temperature, and appetite: a human randomised crossover trial
Does all animal sleep for same/similar duration ?
Not all animals sleep for the same amount of time. Some animals like elephants barely sleep, while others like bats can snooze for almost the whole day. It can also depend on the animal, like horses sleep standing up and dolphins sleep for short intervals alternating one side of their brain and the other side. Even within the same species, it can vary depending on the age or health of the animal. Like baby animals tend to sleep more than grown-ups and sick animals may sleep more than healthy ones.
Table Source : How Much Do Animals Sleep?
A sleep cycle is a recurring period of sleep during the night. The typical sleep cycle lasts around 90 minutes and includes several distinct stages of sleep, including light sleep, deep sleep, and rapid eye movement (REM) sleep. During the cycle, the brain and body progress through these stages in a predictable pattern, with the amount of time spent in each stage varying depending on the stage of the cycle. The first sleep cycles of the night typically have longer periods of deep sleep, while later cycles have more REM sleep. The overall length of the sleep cycle can vary depending on the individual and their sleep needs
Following diagram shows the overall sleep cycle we go through (assuming that we have several hours of good sleep :). Giving you just a big picture of this diagram :
Image Source : Brain Activity During Sleep
Following shows the changes of EEG(brain wave), EMG(Muscle Activity), EOG(Eye Movement) during the sleep cycle.
Image Source : Sleep Disorders: Psychiatric Aspects EEG : ElectroEnchphalogram EMG : ElectroMyogram EOG : Electro-oculography
The sleep cycle is often divided into 5 stages as follows. Try to correlate this description with the illustration shown above and then try to memorize illustrations rather than memorize the verbal description if you have more of vision oriented brain like me.
The main difference between N1 and N2 is the level of brain activity and how easily the person can be awakened. The person is in a deeper sleep in N2 than N1 and it's harder to wake them up. The body is also in a more relaxed state during N2 than N1. Brain activity during NREM sleep, which includes stages N1, N2 and N3, is characterized by slower, synchronized electrical activity, while during REM sleep, the brain's electrical activity is more similar to that seen during wakefulness. During NREM, the brain is less responsive to external stimuli and the body is relatively still, while during REM, the brain is more active, with rapid eye movements, increased heart rate and respiration, and muscle paralysis. The majority of dreaming occurs during REM sleep, that's why it's also called as "Dream Sleep".
Reference
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