The point of sleep, or, Do fruit flies dream of six-legged sheep?

By Ed Yong | August 9, 2008 12:00 pm

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Feeling exhausted after a long day is an all too familiar part of modern life. We drag ourselves into bed, hoping to shut down our minds for a night, waking up recharged the next day. But contrary to popular belief, your brain does anything but shut down during sleep.

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Science is beginning to reveal that sleep is a crucial chance for the brain to consolidate the massive amount of sensory information it receives during the day. It acts as a time-out between periods of consciousness and gives the brain a chance to weave lasting memories from experiences.

For something that is so crucial to our survival, the purpose of sleep is still an enigma to science. It is not simply a question of conserving energy – after all, continually eating doesn’t make you feel any more rested. Now, Indrani Ganguly-Fitzgerald and colleagues from the Neurosciences Institute, San Diego, are drawing back the curtain on the function of sleep. And they are doing it by studying that animal so favoured of geneticists, the fruit fly, Drosophila melanogaster (below).

Flies are not known for their smarts, and at first glance, using them to gain insight into the human mind seems like studying a computer by watching an abacus. But at a fundamental level, the minds of flies and humans work in very similar ways, making them ideal for experimentation.

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For a start, it is very clear that fruit flies sleep. They show distinct periods of rest, when they are unresponsive to the world around them, and they need more rest if they are deprived of it. They also respond to rich environments similarly to humans by increasing both the size of specific brain regions and the number of connections per nerve cell.

Ganguly-Fitzgerald found that the amount of time that flies spend asleep increases with the amount of experience they acquire while awake. She captured adult flies as they emerged from their pupae and twenty-four of these were thrust, like young debutantes, into rich social environments with at least 30 other peers. Five days later, these social flies were sleeping for about twice as long as loner flies who were kept in solitary confinement. They also preferred more substantial hour-long dozes while the loners were content to catch 15 minute catnaps.

Other experiments confirmed the link between experience and sleep. The more peers the networkers mingled with, the longer they needed to sleep. Light snoozers could be converted into heavy sleepers by giving them more social contact and vice versa And mutant strains that were unable to see or smell received no stimulation no matter what circles they mingled with and always slept for the same amount.

These results could be explained by saying that flies which are more active during waking hours consume more energy, become more tired and need longer to recover. But this was not the whole story. By comparing the brains of different flies, the researchers found that the heavy sleepers had three times as much dopamine – a molecule with roles in memory formation – as the light sleepers.

To test the link between sleep and long-term memory, the team trained amorous flies to be uninterested in the opposite sex, by having their advances repeatedly thwarted by unreceptive females. Sure enough, compared to untrained and still-lustful flies, the trainees slept for 40% longer, but not if they were deprived of sleep after their ‘lessons’.

Clearly, snoozing isn’t just a chance for the flies to recuperate after a long day. It allowed them precious time to convert their fleeting experiences into long-lasting memories, and it is likely that the same thing happens in humans. We may even discover the origin of dreaming in the activation of mental circuits as the brain plays back the day’s activities during the night. Tonight, as you drift off to sleep, you do so not to the jumping of sheep, but to the firing of neurons.

Reference: Ganguly-Fitzgerald, Donlea & Shaw. 2006. Science 313: 1775-1781.

Comments (10)

  1. Pierce R. Butler

    … the team trained amorous flies to be uninterested in the opposite sex, by having their advances repeatedly thwarted by unreceptive females.
    Uh, in my personal research, such an experience does not reduce interest in the opposite sex. No sleep logs were kept, but the working hypothesis is that any increase in sleep time was indirect, mediated by ethanol exposure during and following aforesaid experience(s).

  2. chemniste

    Hmm, I wonder if this would work for people. Are there any studies of sleep pattarns of, say, people in prison or in hospital? Perhaps people need to sleep less when they go on those drug trials which require constant supervision.
    Of course it’s complicated in the case of humans because socialising isn’t our only form of stimulation. Other forms of stimulation such as TV, music, reading and so on could result in people needing more sleep to process what they’ve experienced. Were there any attempts to stimulate the flies by other means, such as constantly aggravating them?

  3. Nice – check this as well. I’ll let Indrani know about this post.

  4. Unless sleep is actually the baseline default, and awakened-ness is the aberration or chance mutation of conscious state that evolved into the distinct economic advantage claimed by most animals.
    If aliens landed, we might be surprised to find they’re all asleep, never having accidentally evolved a state we refer to as ‘awake’.

  5. Matt

    This is very interesting. Does this mean that people who are “heavy sleepers” (sleep for longer periods of time than most people) experience more while awake than people who are light sleepers? Or maybe that the heavy sleepers think more than the light sleepers while awake which would make them more tired and want to sleep to consolidate all those memories and thoughts?
    I’d be very interested if you were to follow this article up with some further research in this subject.

  6. yogi-one

    Hmmm… Lay person here, so I have only anecdotal observations, but they seem to support the indications of the study.
    Everyone I have known who does not sleep normally has been mentally off balance. The worst offenders were manic-depressives (stay up three or four days then crash 18 hours), a person I was close to who had progressed schizophrenia (total irregularity of sleep patterns, and not sleeping at all during intense schizophrenic episodes), and depressed people (who can’t sleep because they keep going over disturbing thoughts repetitively that keep their stress levels off the hook). I would suspect that PTSD people, and sleep apnea people also have sleep irregularity connected to their conditions.
    My non-scientific, experienced-based conclusion: not sleeping normally is not good for you.
    There are also people who stay up all night before some important event, like a performance (I myself am a musician) or a big test, thinking that if they stay awake the whole time they will be more alert and do better.
    That doesn’t work. The only time I stayed up all night before exams in college, my instructor re-tested me because he realized I was not functioning normally, and my performance was way below what I usually did in class or on homework assignments.
    I know from experience that I do not play music better by depriving myself of sleep. I will learn new concepts in music and new repertoire faster and more completely if I practice it several times over several days with normal nights of sleep in between than if I try to cram it all in one super-session the day before (or day of) the performance.
    My (again non-scientific, experienced based) conclusion is that this is because music demands both left-brain and right-brain functioning (i.e.- the structure must be understood and mastered, but the piece must be played with intuition and feeling). Sleep seems to co-ordinate the different aspects of the learning, and allow me to feel more intuitive during my performance.
    It will be very interesting to see what further studies indicate.
    Personally, I am still shocked when I hear someone say that think sleep is totally wasted time. For one thing, many studies have already debunked this notion. I myself am hesitant to engage such people in projects, because from what I have seen, if they are denying themselves sleep, both their performance of their responsibilities, and their mental perceptions will be compromised.
    One other thing shocks/disgusts me as well. It seems that in some professions where the stress levels can be extreme, and people’s lives can hang in the balance, we expect people to work 12 and fifteen hour shifts, sometimes with only a few hours between them. I am thinking of emergency room doctors and air-traffic controllers. If you have been in a critical accident, why should your life hang on the judgment of a sleep-deprived, caffeined-out person? Ditto for landings after long flights in stormy weather.
    Thinking that sleep-deprivation is macho is right up there with bragging about how much alcohol you can drink and still drive. Yet there are people who cling to these stupid notions.

  7. @Ian: I can’t tell if you’re joking, but if you’re not then I have to say that strikes me as exceedingly unlikely. Given that so many activities that we require for survival (finding, acquiring, and ingesting food; finding mates and mating; building shelters; caring for offspring; I could go on and on) are only ever done when awake, it seems rather that sleep is just a way that’s evolved to keep the brains we use for all those other activities functional.

  8. @yogi-one: sleep apnea people also have sleep irregularity connected to their conditions.
    Um.. sleep apnea is a condition wherein the person stops breathing while asleep, which wakes them up, sometimes enough that they notice, sometimes not, but generally enough to cause them not to be properly rested. So, yes, they do have “sleep irregularity connected to their condition,” but not in the way you imply (i.e. not in a way that’s parallel to your other examples).

  9. Ian

    Great blog title!

  10. Ramel

    So the short version is that sleep exists for batch processing of sensory data into memory?
    Also the post title rocks.

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