Neuro Science  






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

  • Close eyes. Really, does everybody closes eyes whenever they sleep ?
  • Voluntry Muscle activity reduced. Voluntary muscle activity would not completely shut down, but we may agree that the muscle activity would not be as high as we are awake. Note that the word 'valuntary' is important. That is, the muscle acticity reduction happens to voluntary mustle (like muscles in arms, legs etc) and would not be obvious about non-voluntary muscles like muscles of heart.
  • Go through a specific stage during which eye movement gets very rapid. This is called REM(Rapid Eye Movement) stage, but I don't think you feel / observe this on your own. Does every sleeping animal through this REM stages ?
  • Shows a specific brainwave patterns


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 ?


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.

  • Neural changes like Brain wave changes
  • Physiological changes like changes in a certain hormones
  • Checking the response to stimulus
  • Observing the activity

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 ?


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.

  • Memory Consolidation
  • Reducing Energy Consumption and reserving energies for other vital functionalities
  • Cleaning up the metabolic waste (e.g, Cleaning up Glymphatic System) - Why Sleep is Important



Impact on Memory Process


Following is an example on how sleep impact on Memory Process. As shown in [a], you would notice :

  • You would have higher recognition performance as you have longer slow-wave-sleep (NOTE: slow-wave-sleep implies deeper sleep. Refer to the note on sleep cycle to get further details on sleep cycle).
  • Memory process improvement with sleep is observed more drastically for remote memory (NOTE : recent memory refer to the memory formed recently and remote memory refer tos the memory formed relatively long time ago).

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.



Impact on Energy Consumption


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?

Species Average Total Sleep Time
(% of 24 hr)
Average Total Sleep Time
Brown Bat 82.9% 19.9 hr
Giant Armadillo 75.4% 18.1 hr
North American Opossum 75% 18 hr
Python 75% 18 hr
Owl Monkey 70.8% 17.0 hr
Human (infant) 66.7% 16 hr
Tiger 65.8% 15.8 hr
Tree shrew 65.8% 15.8 hr
Squirrel 62% 14.9 hr
Western Toad 60.8% 14.6 hr
Ferret 60.4% 14.5 hr
Three-toed Sloth 60% 14.4 hr
Golden Hamster 59.6% 14.3 hr
Platypus 58.3% 14.0 hr
Lion 56.3% 13.5 hr
Gerbil 54.4% 13.1 hr
Rat 52.4% 12.6 hr
Cat 50.6% 12.1 hr
Cheetah 50.6% 12.1 hr
Mouse 50.3% 12.1 hr
Rhesus Monkey 49.2% 11.8 hr
Rabbit 47.5% 11.4 hr
Jaguar 45% 10.8 hr
Duck 45% 10.8 hr
Dog 44.3% 10.6 hr
Bottle-nosed dolphin 43.3% 10.4 hr
Star-nosed Mole 42.9% 10.3 hr
Baboon 42.9% 10.3 hr
European Hedgehog 42.2% 10.1 hr
Squirrel Monkey 41.3% 9.9 hr
Chimpanzee 40.4% 9.7 hr
Guinea Pig 39.2% 9.4 hr
Human (adult) 33.3% 8 hr
Pig 32.6% 7.8 hr
Guppy (fish) 29.1% 7 hr
Gray Seal 25.8% 6.2 hr
Human (elderly) 22.9% 5.5 hr
Goat 22.1% 5.3 hr
Cow 16.4% 3.9 hr
Asiatic Elephant 16.4% 3.9 hr
Sheep 16% 3.8 hr
African Elephant 8.3% 2.0 hr
Donkey 13.0% 3.1 hr
Horse 12.0% 2.9 hr
Giraffe 7.9% 1.9 hr




What is sleep cycle ?


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 :

  • Each of the cycles spans around 60 to 90 mins
  • The first two cycles goes through the deepest cycle (i.e, up to Stage 4).
  • At some point of most cycles, we go through REM(Rapid Eye Movement) sleep
  • (In general) As we go through deeper stages of sleep, the frequency of the brain tend to get lower (i.e, slower brain wave)

Image Source : Brain Activity During Sleep



Following shows the changes of EEG(brain wave), EMG(Muscle Activity), EOG(Eye Movement) during the sleep cycle.

  • EEG Changes in Sleep Stages
    • During NREM sleep(Stage 1,2,3,4), EEG readings show slower, higher amplitude brain waves, while during REM sleep, EEG readings show faster, lower amplitude brain waves
    • Among the stages during NREM sleep, EEG reading shows slower and higher emplitude as the stage goes deeper.
  • EMG Changes in Sleep Stages
    • During NREM sleep, muscle activity continues to decrease as the person progresses through the stages of N2 and N3. In N2, muscle activity is more synchronized, while in N3, muscle activity is at its lowest level.
    • During REM sleep, EMG readings show low levels of muscle activity, which is known as atonia. This state of muscle relaxation is thought to prevent the person from acting out their dreams, which can be vivid and sometimes disturbing.
  • EOG Changes in Sleep Stages
    • During NREM sleep, eye movement continues to decrease as the person progresses through the stages of N2 and N3. In N2, eye movement is less frequent, while in N3, eye movement is almost absent.
    • During REM sleep, EOG readings show rapid eye movement (REM) as the eyes rapidly move in different directions. This is a characteristic of the REM stage, where vivid and sometimes disturbing dreams occur.


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.

  • N1 : Stage 1 (light sleep). Stage N1, also known as drowsy sleep or light sleep, is the stage between being fully awake and falling asleep. During this stage, the brain produces alpha and theta waves, which are slower and of lower amplitude than the beta waves produced during wakefulness. Muscles begin to relax and the person may experience hypnic jerks or sensations of falling. The eyes are closed but the person is easily awakened. This stage usually lasts for 5-10 minutes
  • N2 : Stage 2 (light sleep). Stage N2, also known as light sleep, is the first stage of true sleep. The brain produces sleep spindles and K-complexes, which are brief bursts of activity that help to protect the sleeper from external stimuli. The person's heart rate and breathing begin to slow down, and the body temperature drops. This stage usually lasts for around 20-25 minutes and it makes up around 50-55% of an adult's sleep time. It is believed that Sharp Wave and Riiples (SWR) primarily occurs at this stage.
  • N3 : Stage 3 (deep sleep). Stage N3 is also known as slow-wave sleep (SWS) because of the slow, large amplitude brain waves that are present during this stage.
  • N4 : Stage 4 (deep sleep). N4 is a stage of deep sleep that is also sometimes referred to as delta sleep. It is characterized by even slower brain waves, called delta waves, which can reach as low as 0.5 Hz. This stage of sleep is considered the most restorative and restful stage of sleep, and it is during this stage that the body releases growth hormone and repairs muscles and tissues. During stage N4, it's hard to wake up a person, and if they are woken up, they will feel groggy and disoriented.
  • REM (Rapid Eye Movement). As the name suggests, REM sleep is characterized by rapid, random movement of the eyes. The detailed activities during REM sleep is described as below.
    • Low muscle tone: During REM sleep, muscle tone decreases, leading to a temporary paralysis of the body's skeletal muscles, known as atonia. This is thought to prevent the person from acting out their dreams.
    • Increased heart rate and breathing: Heart rate and breathing increase during REM sleep, which may be related to the increased brain activity that occurs during this stage.
    • Brain activity: REM sleep is characterized by increased brain activity in the areas of the brain responsible for processing emotions, memories and learning.
    • Dreams: REM sleep is associated with vivid and sometimes disturbing dreams

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".