Why We Sleep, Revisted

By Neuroskeptic | May 16, 2012 5:15 pm

I’ve got another guest post over at Discover magazine: Is the Purpose of Sleep to Let Our Brains “Defragment,” Like a Hard Drive?

It’s an expanded version of two Neuroskeptic posts(1,2) about the theory that the job of slow-wave sleep is to prune connections in the brain, connections which tend to become stronger while we’re awake and might become too strong without periodic resetting.

One of the commenters on the Discover post pointed out that this idea a bit like a much older idea about sleep, from Francis Crick (of discovering-the-structure-of-DNA fame). Back in 1983, Crick and Graeme Mitchison proposed that dreaming sleep serves to help us “unlearn”: The Function of Dream Sleep
Their idea was a bit different, but it was really very elegant.

The sleeping brain, they said, is cut off from real sensory input, and is subject only to essentially random activity variations. However, sometimes these meaningless inputs may be ‘interpreted’ as having meaning, activating representations (concepts, thoughts, memories) that we’ve learned to recognize when awake.

These “patterns” that the brain wrongly “sees” in noise are what we experience as dreams. Crick and Mitchison’s point is that, ideally, a pattern recognition system (like the brain) shouldn’t be picking up patterns from random noise because that would be a sign that it was biased in favor of those patterns – “obsessed” with them, as it were, and liable to see them everywhere. So it would be good if there were some way of identifying the brain’s overlearned biases and (partially) unlearning them.

That’s what dreams do, somehow, according to Crick. It’s like if dreams are a self-administered Rorschach test that the brain uses to work out what’s “weighing on its mind”! Incidentally, this is an idea I once suggested (much less clearly) myself.

It’s a beautiful and ingenious theory, although as the authors admitted, it would be very hard to test and it leaves wide open the question of how dreams could cause memories to be “unlearned” or indeed whether unlearning is even possible in the brain. It’s also not very similar to the modern “defrag” theory, because Crick was talking about the dreaming rapid eye movement stage of sleep, not slow-wave sleep.

ResearchBlogging.orgFrancis Crick and Graeme Mitchison (1983). The Function of Dream Sleep Nature, 304, 111-114

CATEGORIZED UNDER: blogging, links, papers
  • http://www.blogger.com/profile/05109730195575401381 A_Mitchell

    Dreams happen because the brain thinks it's awake (same brainwave pattern, or Crick's 40 Mz) and seeks input without realizing (except when lucid) that it's not externally available. It ramps to 40 Mz because it has to process sense experience into long term memory, and it's best to do this in similar conditions to when the experiences were encoded (i.e., while awake), due to something called encoding specificity. You heard it here first.
    – Mitch Moldofsky, @thinkingskillsc

  • DS

    Neuroskeptic wrote:

    “It's a beautiful and ingenious theory, …”

    How is this a theory? I don't even see a testable a hypothesis.

    A beautiful story perhaps.

  • http://www.blogger.com/profile/10982658260354790408 retrun

    I think proposing high level theories about why we (as well as other mammals and birds) dream is useful for arguments sake, but I'm not convinced that we really know how a single neuron really works, let alone a huge collection of them.

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Story seems a bit harsh. I'd say it was an account at least. Maybe a model even. 😉

  • NoOneSelf

    I always wince when I see people talk about “purpose” in biological systems. Sure, it might just be a convenient convention of language, but I can't help but think it may constrain or even misdirect our attempts to fully understand those systems. Thoughts?

  • http://www.blogger.com/profile/05109730195575401381 A_Mitchell

    Sorry – biological systems shouldn't have a purpose? Do you mean in a Darwinian way, i.e., no ultimate, ordained purpose, or just a functional way? I can see the former but need some explanation if the latter.

  • Ivana fulli MD

    DS said…16 May 2012 20:01

    /// How is this a theory? I don't even see a testable a hypothesis.

    A beautiful story perhaps.///

    How right can Neuroskeptic be to correct you politely and call it a model.

    1) Are you so proud of Pr Stahl and followers simplistic monoaminergic hypothesis of mental disorders, DS?

    2) Francis Crick had been harshly treated by DS minded fellow scientists when he called “central dogma” the idea of an information track going mostly one way : DNA ? RNA ? Protein, before he demonstrated it with colleagues.

    He should have called it a “The ultimate molecular biology model” or whatever.

    It was a “premature dogma” for sure- although for a short time in science history.But he had a great idea and was proven right!

    So what DS? Francis Crick was not a modest man but he had not much to be modest about (to paraphrase Churchill stating that Adlee was a modest man with much to be modest about)!

    May I suggest you read Crick's book “The Astonishing Hypothesis” . It is about neuroscience now able to begin a scientific study of how brains produce conscious experiences if only neuroscientists could work in teams with different disciplines and not use computational models of mental function devoid of sound “hypothesis on brain structure and function- for what I understand.

    PS: I am a nobody in research but I had a good education in many ways and from 1997 to 2001 I did some informal pro bono work as a psychiatrist to sustain a few parents of an association of parents whose very highly gifted children were suffering hell at school. Just a passing thought-not a hypothese, DS: Very often the most gifted and creative children and teenagers could be called arrogant-but all had been bullied heavily at school and were bored at school:they felt that their bullies were thick. The brightest often were considered academic failures by primary or secondary teachers of medium range intelectual abilities to remain polite. NB: F Crick was not accepted at Cambridge -GB- as an undergrad student and was refused the genetic chair he had wanted at some point. Nothing to be proud about by that university.

    Anyway, the important thing is to get more Cricks being involved in neurobiology and, frankly- as petrossa often point out- a new Francis Crick would have to be modest and to tolerate fools lightly in order to listen to an average neuroscience meeting in 2012…

  • Anonymous

    Right and if we were too obsessed with what was bothering us in our daily life, we would ruminate and never get to sleep. Which is why, as Freud pointed out long ago, we dream in order to sleep!!!

  • Anonymous

    Freud was right!

  • Ivana Fulli MD

    Anony mouses 17 May 2012 12:44 and 17 May 2012 13:30:

    ///As Freud pointed out, long ago we dream in order to sleep!!!///


    ///Freud was right!///

    Please anonymouses, tell us where did Sigmund Freud ever write or say that the purpose of sleep were to let our brains defragment like-a hard-drive and the job of slow-wave sleep to prune connections in the brain, connections which tend to become stronger while we're awake and might become too strong without periodic resetting?

  • omg

    Apparently when I was born to when I walked I slept so much it was ridiculous. How much content does a baby have that it has to sleep so much to defragment its daily life ? Why do we sleep less with age ?

  • Anonymous

    Well excuse me Ivana Fulli:
    I didn't say that Freud wrote of brain defragmenting or pruning. You apparently have difficulty reading English: I simply wrote that Freud's basic point about the function of dreams was that they enabled the organism to sleep and sleep is a restorative process.Please keep your criticism to what I actually wrote and not what you think I wrote. Thanks.

  • http://www.blogger.com/profile/02537151821869153861 Andrew Oh-Willeke

    From a neurological activity and relationship of sleep to learning perspective the defrag theory makes sense. But, it doesn't seem to provide a very useful account of what mental functions tend to fail in what order in people experiencing sleep deprivation (e.g. people cease to attend to routines but handle higher order tasks much longer).

    It also seems to put too much emphasis on human specific processes that take place during sleep, when sleeping is ubiquitous for mammals and probably many other vertebrate species as well. If all mammals need sleep then the necessity should arise from the limbic system and not the activitites of the cortex.

  • Ivana Fulli MD

    Anonymous said… 17 May 2012 17:44

    ///You apparently have difficulty reading English:///

    1) Your little piece of xenophobia is to my mind the equivalent of a Godwin point….

    2) ///I simply wrote that Freud's basic point about the function of dreams was that they enabled the organism to sleep and sleep is a restorative process.///

    Freud basic point in writing -and rewriting up to 8 editions of his great work – on interpretation of the dreams was to interpret the dreams as a way to reach the inconscious!

    Being xenophobic against my poor command of English will not go far there since Freud wrote in German my dear…

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    Right, that's enough of this argument, Ivana Fulli MD & Anonymous. Debating Freud is fine, but don't insult each other.

  • http://survivingantidepressants.org Iatrogenia

    Hmmm….I'm wondering if the supposed neurogenesis attributed to antidepressants isn't failure to prune, as they screw up sleep architecture as well.

  • Anonymous

    I apologize to Dr. Fulli.

  • Ivana Fulli MD

    Anonymous said…

    I apologize to Dr. Fulli.

    21 May 2012 02:03

    No need for apologies: I am the one who gets on Neuroskeptic nerves!

  • Ivana Fulli MD

    Iatrogenia said…

    Hmmm….I'm wondering if the supposed neurogenesis attributed to antidepressants isn't failure to prune, as they screw up sleep architecture as well.

    20 May 2012 18:24

    Well put!

    Also, one wonders how a foetus can sleep in the womb when inundated with antidepressants taken by the innocent mother who is being told that those drugs will produce a healthier baby when “depression” of the pregnant mother will be catastrophic for the new born…

  • http://www.blogger.com/profile/06647064768789308157 Neuroskeptic

    That's a very interesting idea about antidepressants actually. I'd not thought of that.

  • Ivana Fulli MD


    21 May 2012 19:24:/// That's a very interesting idea about antidepressants actually.///

    It might be an argument about not prescribing -whenever possible- SSRI before “brain adulthood” (22 years of age ?)and makes one pay attention to the November 1, 2011 article in the Archives on :

    Antidepressant Use During Pregnancy and Childhood Autism Spectrum Disorders

    Arch Gen Psychiatry. 2011;68(11):1104-1112. doi:10.1001/archgenpsychiatry.2011.73

    Out of topic but worth taking also into account for the prescriber: the twofold increase in persistent pulmonary hypertension of the new born (from 3 to 6% incidence in the Northern country study).

    Selective Serotonin-Reuptake Inhibitors and Risk of Persistent Pulmonary Hypertension of the Newborn — NEJM

    Selective serotonin reuptake inhibitors during pregnancy and risk of persistent pulmonary hypertension in the newborn: population based cohort study from the five Nordic countries | BMJ

    NB:many autistic infants and children are very bad sleepers sleepers-one main complain from the parents- but withdrawal symptoms from SSRI include for some people terible insomnia for years sometimes.

    On the other hand, many autistic children without previous SSRI impregnation through the placenta are bad sleepers. But here one would want them to be helped to sleep better using what we know about hypersensitivity to light, sounds, smells( be it for pruning or whatever reasons). A doctor should be able to prescribe bedroom fitting and the like …

  • http://survivingantidepressants.org Iatrogenia

    I'm very, very skeptical about the supposed benefit of antidepressant-induced neurogenesis, if in fact it exists.

    This paper suggests the stress of withdrawal wipes out such neurogenesis: Biol Psychiatry. 2003 Nov 15;54(10):1105-17.
    Neurobiology of antidepressant withdrawal: implications for the longitudinal outcome of depression.
    Harvey BH, McEwen BS, Stein DJ.

    Re neonate withdrawal syndrome — in adults, hypersensivity to sounds, etc. is common in severe antidepressant withdrawal syndrome, as is sleeplessness and surges of anxiety in the morning matching the a.m. cortisol peak.

    These symptoms can last for years.

    In these situations, we recommend reducing light stimulation. Blackout curtains and wearing a sleeping mask in the bedroom reduce early-morning waking with panic, and wearing blue-blocking glasses or sunglasses reduce daytime stimulation. Computer use at night is restricted.

    This enables people to cope better with their hyper-alerting symptoms.

  • Ivana Fulli MD


    Thanks for your contribution and testimony. You are doing a very good work by raising awareness about antidepressants withdrawal symptoms and by offering support and advices in withdrawal.

    We, psychiatrists were told withdrawal from antidepressant didn't exist etc…

    Still, some autistic persons -like petrossa who frequently comment here and is a very articulate and intelligent commentator -want antidepressant and/or antiepileptic prescriptions because they suffer on the sensory front something looking a lot like antidepressant withdrawal symptoms.

    PS: Please do not take me for a drug pusher. I am not: I did my MD thesis on cases of benzo pseudo-neurosurgical side-effects at the beginning of the eighties and I went as far as asking -with little success so far- the Paris medical board in 2008 to let me open a homeopathic prescribing only (and support and psychotherapy)psychiatrist office because I find the overpresciption of Big Pharma drugs a sanitary scandal.

    Still, some people feel obviously helped by those drugs and not everybody suffer the same pain in withdrawal.

    Just to disregard those persons -neuroskeptic being one- doesn't make sense to my mind.

    We need to know more about it in order to know who can be helped by those drugs.

  • http://survivingantidepressants.org Iatrogenia

    Ivana, Thank you for the compassion you express above. If you know how to very gradually taper patients off psychiatric drugs over months (or know any other MDs who do this), please contact me at survivingads at comcast dot net.

    Injury from abrupt withdrawal is still frequent and patients need to find doctors who know how to taper.

    To your other points, I didn't say a word about banning antidepressants.

    Please look into your own over-reaction to truthful exposure of an under-recognized risk.

    The risk-benefit of a psychiatric drugs has to be assessed for each individual based on an honest acknowledgement of risk. Medicine is a long, long way from that honest acknowledgement, as so many resources have been devoted in the last 20 years to obscuring the risks.

    If new information about risks suggests to you fewer people should be taking these drugs, that seems to me a responsible position, rather than a tragedy.

    PS I believe more shoes will be dropping re maternal medication and autism.

  • Ivana Fulli MD


    My main point here was just to put some emphasis on a subject I am apparently alone at finding fascinating :

    Some autistic persons having never had any antidepressant treatment of any sort suffer from symtoms very similar ot those found in antidepressant withdrawal.

    And also to remind you that some people are just happy and grateful to take antidepressants like Neuroskeptic and petrossa.

    NB: I am based in France and I am afraid that giving you adresses on that website of yours will not help much since tapering /replacing antidepressants during a French holiday is not really a rerasonable option.

    You might not like either the fact that I found homeopathy helpful- sometimes a lot- when a person agrees to let go of antidepressants.

    Some people see directly an homeopath and escape the antidepressant prescription.

    Also, hormones sometimes help a lot if you can find a gynaecologist agreing to help and not only for post-menopausal women.

  • http://survivingantidepressants.org Iatrogenia

    Ivana, I have members in Europe and am collecting contact information for doctors all over the world who will attend to an individualized, gradual taper off psychiatric drugs.

    A gradual taper may not be accomplished over a French holiday, it may take a year or more.

    Again, I do not advocate banning antidepressant treatment. Please examine why you persist in that reading of my statements.

    I don't force or recruit anyone to abandon medication, I only help those who seek it.

    As for the autistic: My points are 1) The autonomic dysfunction suffered in severe antidepressant withdrawal by adults includes sensory hypersensitivity that may be similar to that experienced by an autistic child.

    2) The association of maternal antidepressants with autism in the children may be via severe withdrawal syndrome mechanisms in the neonate.

    I don't want to scare you too badly, but my expectation is that in the near future, for this and other reasons to protect the health of the neonate, medicine will restrict the prescription of psychiatric drugs for pregnant women and those planning to become pregnant.

  • http://www.blogger.com/profile/12033918835169823548 MJ

    If what you are saying is true, I wonder what if it could explain some of the neural connectivity problems sometimes seen in autism.

    If regular sleep is needed to help the brain organize itself, does that mean that the chronic sleep problems that so many people with autism have might lead to problems in connectivity that are also quite common?



No brain. No gain.

About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.


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