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. 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 check if an animal is in sleep or not.

  • Brain wave
  • 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.




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


Image Source : Declarative memory consolidation in humans: A prospective functional magnetic resonance imaging study




Does all animal sleep for same/similar duration ?



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 ?



Image Source : Brain Activity During Sleep



Image Source : Electroencephalogram Mapping of Brain States

EEG : ElectroEnchphalogram

EMG : ElectroMyogram

EOG : Electro-oculography