I was just reading the sleep threads, and I wondered, why is the body so constructed so that sleep is necessary? (Is it just a design error?) But then, how do things come to be awake at all? How did this happen? Yes, I can see the advantages, but how did it happen? What is "awake"? Are single-cell animals awake?

The question I want to ask is, how did things progress from just sitting around ingesting and growing, like plants, to being something more? Is it more? What do "wakers" have that the rest don't?

Sorry, I'm not a biologist, this may be a foolish question, or in the wrong place.

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    $\begingroup$ Hi and welcome to the Biology part of SE! I think you're tying in a few different concepts here, and they're really more philosophical than biological. I don't think it makes for a good SE question, but feel free to join me in Chat and I can talk you through some of the basics that I'm aware of. $\endgroup$
    – Armatus
    Sep 10, 2014 at 11:54
  • $\begingroup$ We should be fine using this: chat.stackexchange.com/rooms/1997/biology $\endgroup$
    – Armatus
    Sep 10, 2014 at 11:55
  • $\begingroup$ That chat room won't let me in. Never mind. But if you want to copy my question I could follow other people chatting about it... $\endgroup$
    – RedSonja
    Sep 10, 2014 at 12:13
  • $\begingroup$ these are fundamental questions in sleep research done by neuro/sleep biologists... probably a field which is poorly represented in the stackexchange community. Its a common question about sleep, but also regarded well enough for it to be funded by the NIH for research... $\endgroup$
    – shigeta
    Sep 10, 2014 at 17:13

2 Answers 2


Based on various comments, I've expanded this answer. Though long, I cannot cover the depth of the topic of sleep but I tried to address the issues asked in the original question while broadly highlighting various aspects about sleep research. I welcome any suggestions for improvement.

What is sleep?

To know what it means to be awake, you must know what it means to be in a state of sleep. These are two alternative states for animals that sleep.

Finding a concise scientific definition of sleep is difficult, probably because sleep is a very complex process. For example, Diekelmann and Born (2010) state that,

Sleep has been identified as a state that optimizes the consolidation of newly acquired information in memory...

and later

Sleep is characterized by the cyclic occurrence of rapid eye movement (REM) sleep and non-REM sleep, which includes slow wave sleep (SWS, stages 3 and 4) and lighter sleep stages 1 and 2....

Most attempts to explain what sleep is include the four stages of sleep along with REM (rapid eye movement sleep). The stages are nicely explained by this National Institutes of Heath website on Brain Basics: Understanding Sleep. Each stage of sleep can be identified by specific types of brain waves, shown below (image from here).

Comparison of brain wave activity in the states Awake, Drowsy, Stage 1 sleep, Stage 2 sleep, Slow-wave sleep (stage 3 and stage 4 sleep), and REM sleep

Notice that the first wave pattern is characteristic of being awake. The wave patterns change as an individual becomes drowsy and then falls asleep. While asleep, the brain cycles through the different stages, each with its characteristic brain waves. Associated with these stages are changes in metabolic activity and physical capabilities. For example, the Brain Basics site notes that we become less responsive to external stimuli and metabolic activity slows. During REM, breathing becomes rapid and we can't move the muscles in our arms or legs. Similar states are found in other invertebrate and vertebrate organisms, which suggests that these are common indicators of sleep or a sleep-like state. Some examples of these states are given in the citations and links below.

Defining "awake" seems just as difficult as defining "asleep" but once again neural activity seems to be key indicator. As shown in the figure above, being awake has characteristic wave patterns in the brain that are distinct from the wave patterns associated with sleep. In addition, none of the physiological/physical states associated with sleep stages are present when you are awake.

In both cases, asleep and awake, the indicators seem to be based on characteristic neurological activities in the brain or, in the case of some invertebrtes, the organized nervous system. This interpretation is important when considering whether organisms without any type of nervous system is capable of sleep or the alternate state: awake. More on this at the end.

Why sleep?

Science magazine, in the 125th anniversary issue, made a list of 125 big questions that remain to be answered by scientists. One of those questions was "Why do we sleep?" The role of sleep remains elusive but many ideas have been put forth about the role of and the evolution of sleep. The leading idea seems to be memory consolidation. This article by Kavanau (1997) reviews ideas about the function of sleep and why it evolved. He suggests that sleep and memory evolved as as ways to improve or maintain effective connections among the nerve cells in the brain. Some circuits are used frequently, such as those used to process sensory information like vision. Other circuits are used less frequency, such as circuits that can be used to store memories. Sleep may allow those infrequently used circuits to be activated and used without causing conflict with the circuits used during "restful waking." In other words, sleep may allow the lesser used circuits to be exercised, which can help to consolidate memories. So, at least one function of sleep may be to help maintain efficiency in how information is moved around the brain by maintaining the brain circuitry. See also this review by Siegel (2005) and the previously mentioned article by Diekelmann and Born (2010).

This Scitable article has a nice overview of the importance of sleep for humans, as does the Brain Basics site.

What organisms sleep or have sleep-like activity?

Sleep has been well-studied in vertebrates, from fishes (Marshall 1972) to mammals (Pace-Schott and Hobson 2002). However, not all vertebrates necessarily sleep (Kavanau 1998). For example, continuously swimming fishes like some sharks and blind cave fishes may not sleep. These are just three references of many about sleep in vertebrates.

Sleep and sleep-like activity has not been as well studied in inertebrates. As noted in this thread, there is evidence for sleep in at least some insects. Cephalopods like octopuses show signs of sleep (Mather 2008). Caenorhabditis elegans, a nematode worm and model biological system, shows sleep-like states tht are similar to mammals and fruit flies (Raizen et al. 2008). The most basal organism that shows evidence of sleep-like patterns is a jellyfish called Chironex fleckeri (Kavanau 2006). Cnidarians like jellyfishes are the basal (most "simple") group of organisms with an organized nervous system. Interestingly, C. fleckeri process a lot of visual information obtained through their 24 eyes. Twelve of the eyes are simple, light-receiving structures but 8 of the eyes are camera-like, meaning they have a lens. Kavanau (2006) argued that sleep (up to 15 hours) in this jellyfish allows time for its simple nervous system to process the visual information obtained while it is awake. The presence of sleep-like activity in jellyfish and nematode worms suggest that sleep is an evolutionarily ancient phenomenon.

This Wikipedia page on sleep in non-humans overviews evidence for sleep in vertebrates and invertebrates. It's not clear whether androids sleep or what they dream about if they do sleep.$^1$

Do single-celled organisms sleep?

This is (to me) a tricky question. Sleep is associated with specific types of neural activity in the brain or nervous system. In addition, sleep-like neural activity has been found in diversity of organisms with some type of nervous system, and seems evolutionarily old. I therefore think that any organism that has some form of organized neural center (brain, brain-like organ or centralized nervous system) has the potential to show some type of sleep-like patterns.

In contrast, organisms lacking any type organized nervous system would not be capable of sleep, at least as currently defined. If they do not sleep, then they also cannot be awake because being "awake" seems to be recognized as neural activity not typical of sleep. Therefore, it seems to me that single celled organisms like bacteria and paramecium are neither sleeping nor awake. Plants are neither sleeping nor awake. They are metabolically active but they are neither asleep nor awake because they do not have any type of neural activity.

I'm not a sleep researcher (but sometimes I am a sleepy one) nor am I a neurophysiologist. I just walked through my train of thought. I welcome contrary views along with the supporting science.

Literature Cited

Diekelmann, S. and J. Born. 2006. The memory function of sleep. Nature Reviews Neuroscience 11: 114-126.

Kavanau, J.L. 1997. Origin and evolution of sleep: Roles of vision and endothermy. Brain Research Bulletin 42: 245-264.

Kavanau, J.L. 1998. Vertebrates that never sleep: Implications for sleep's basic function.

Kavanau, J.L. 2006. Is sleep's 'supreme mystery' unraveling? An evolutionary analysis of sleep encounters no mystery; nr does life's earliest sleep, recently discovered in jellyfish. Medical Hypotheses 66: 3-9.

Marshall, N.B. 1972. Sleep in fishes. Proceedings of the Royal Society of Medicine (abstract).

Mather, J.A. 2008. Cephalopod consciousness: Behavioral evidence. Consciousness and cognition 17: 37-48.

Pace-Schott, E.F. and J.A. Hobson. 2002. The neurobiology of sleep: Genetics, cellular physiology and subcortical networks. Nature Reviews Neuroscience 3: 591-605.

Raizen, D.M. et al. 2008. Lethargus is a Caenorhabditis elegans sleep-like state. Nature 451: 561-572.

Siegel, J.M. 2005. Clues to the functions of mammalian sleep. Nature 437: 1264-1271.


  1. I could not resist.
  • $\begingroup$ I think your answer can be considered totally complete to this broad question if you could add a bit more about the standard definitions of "sleep" and "awake" in science and precise for which species the concept of sleep applies. $\endgroup$
    – Remi.b
    Sep 10, 2014 at 13:33
  • $\begingroup$ @Remi.b Thanks for the suggestion. I'll try to carve out time later today after my classes. $\endgroup$ Sep 10, 2014 at 13:35
  • $\begingroup$ That article is really interesting. I think you have answered my question, and in doing so, have made it seem more reasonable than it was. I look forward to the changes Remi.b suggests. $\endgroup$
    – RedSonja
    Sep 10, 2014 at 14:00
  • $\begingroup$ So sleep is the brain defragmenting itself. OK. If you don't have any software running you don't need to sleep. OK. And the reason we do it is that evolution threw it at us and it seemed a good idea. OK. I would give you +10 if I could. $\endgroup$
    – RedSonja
    Sep 11, 2014 at 8:00
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    $\begingroup$ @RedSonja Yes, sleeps helps to consolidate memories. Having memories, even very simple ones, may provide an evolutionary advantage (remembering where food resources are, what food is safe to eat, which organisms are dangerous, etc.). You accepted the answer (thanks!) but you can also click on the upward arrow to say "Good answer" if you wish. $\endgroup$ Sep 11, 2014 at 9:02

All organisms (except viruses?) need some kind of nutrition to survive and reproductive activities for the species to survive. As humans don't do these while asleep in a first approximation I would consider those life phases as sleep when these do not happen. In this sense clostridium bacteria could be considered asleep when in their endospore states, a sunflower seed could also be considered asleep as it is inactive.

  • $\begingroup$ So we could say sunflower seeds wake up at some point? $\endgroup$
    – RedSonja
    Sep 10, 2014 at 12:44
  • $\begingroup$ @RedSonja Yes, along the above I would consider the seed awake as soon as it starts using up its stored food. $\endgroup$
    – winerd
    Sep 10, 2014 at 12:46
  • $\begingroup$ I don't think I can see any phase of a plant's life as being awake. No. But then I don't really know what "awake" means. $\endgroup$
    – RedSonja
    Sep 10, 2014 at 12:50
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    $\begingroup$ Sleep is associated with neural activity. Bacteria and sunflowers do not have a nervous system so could not have any form of sleep. Endospores and seeds can be dormant but not asleep. $\endgroup$ Sep 10, 2014 at 12:50
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    $\begingroup$ Sleep includes changes in neural activity patterns of the brain see here. These changes in neural activity do cause changes in metabolic activity, such as rapid eye movement and suspension of voluntary muscle activity. I agree than awake doesn't make sense for organisms that don't have sleep-like patterns but I did not address the philosophical implications of that part of the question. $\endgroup$ Sep 10, 2014 at 13:07

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