Sleep Is To Save Energy

Short sleepers, about 1% to 3% of the population, function well on less than 6 hours of sleep without being tired during the day. They tend to be unusually energetic and outgoing. (more)

What fundamental cost do short sleepers pay for their extra wakeful hours? A recent Science article collects an impressive range of evidence (quoted below) to support the theory that the main function of sleep is just to save energy – sleeping brains use a lot less energy, and wakeful human brains use as much as 25% of body energy. People vary in how much sleep they are programmed to need, and if this theory is correct the main risk short sleepers face is that they’ll more easily starve to death in very lean times.

Of course once we were programmed to regularly sleep to save energy, no doubt other biological and mental processes were adapted to take some small advantages from this arrangement. And once those adaptations are in place, it might become expensive for a body to violate those expectations. One person might need sleep because their bodies expect them to sleep a lot, but another body that isn’t programmed to expect as much sleep needn’t pay much of a cost for that, aside from the higher energy cost to run the energy-expensive brain more.

This has dramatic implications for the em future I’ve been exploring. Ems could be selected from among the 1-3% of humans who need less sleep, and we needn’t expect to pay any systematic cost for this in other parameters, other than due to there being only a finite number of humans to pick from. We might even find the global brain parameters that bodies now use to tell brains when they need sleep, and change their settings to turn ems of humans who need a lot of sleep into ems who need a lot less sleep. Average em sleep hours might then plausibly become six hours a night or less.

Those promised quotes:

Sleep … appears to be nearly universal among birds and mammals, leading to the assumption that sleep serves an unknown but vital physiological function. However, no function that can explain the huge variation in sleep times within and between species has yet been firmly identified, although many candidates, including reversal of oxidative stress, memory consolidation, extension of life span, and removal of various neurotoxins, have been proposed. …

Sandpipers engage in complex courtship displays and aggressive defense of potential mates over 3-week periods. … During this time, the males show no reduction in activity or degradation of performance despite little or no sleep…. Killer whale and dolphin mothers and their calves are continuously active with eyes open for 6 or more weeks after birth. No rebound of inactive behavior follows. During this period, the neonates’ brain and body grow to their prodigious size and capacity without any apparent need for sleep-linked detoxification. Adult dolphins working for reward can accurately discriminate between visual stimuli presented at 30-s intervals on their left or right sides, 24 hours per day, for as long as 5 days. During this time, their performance shows no progressive decline. No rebound of inactivity follows the session. By contrast, humans whose sleep is interrupted on a similar schedule are dramatically impaired . … Migrating birds greatly reduce sleep time with intact learning abilities and high rates of performance, with no subsequent sleep rebound. …

If male sandpipers that are continuously active during the breeding season clearly leave more offspring than males who sleep, why hasn’t natural selection eliminated the sleepier males? An attractive explanation is that the active males are more likely to deplete caloric reserves. … In conditions of food scarcity, the birds that most greatly depleted caloric reserves during mating will be least likely to survive to the next mating season. …

Humans … of similar age, sex, and body build can have very different sleep times. They can also vary in their response to sleep loss, with some being highly impaired, unable to resist sleep, and others showing high levels of functioning despite sleep loss. The effect of sleep deprivation on performance is not strongly related to baseline sleep duration. Furthermore, human sleep duration is not linearly related to health, with both high and low values being linked to shortened life span. Death of rats due to sleep deprivation may be related to stress rather than sleep loss. Sleep deprivation has not been reported to cause death in pigeons, mice, or in rats deprived by techniques that do not involve waking them frequently at sleep onset. Fatal familial insomnia can cause death in humans, but sleep loss does not appear to be responsible. …

So why do most of us feel so poorly when we reduce our sleep time? Natural selection has imposed a certain amount of sleep on us to restrict activity to appropriate times of day and to reduce long-term nonvital energy expenditure. The pressure to sleep operates by reducing brain activity. Although individuals with naturally short sleep times are not at elevated risk compared to those with naturally long sleep times, repeated sleep deprivation below the body’s programmed level is stressful and likely to impair health. Certain hormonal processes are linked to sleep. However, these are not universal, but rather are species and age specific.

Sleep duration varies enormously across species, with total sleep amount ranging from 20 hours per day in the big brown bat to 2 hours per day in the horse. However, attempts to correlate sleep time with various parameters do not support any sleep physiology theory. But species that eat food with low caloric density (e.g., herbivores) sleep less than those eating more nutritionally dense foods (e.g., carnivores) … Animals will achieve a selective advantage in reducing brain energy consumption by sleep, but only if they have safe sleeping sites, such as under-ground burrows. Accordingly, large prey animals that do not have safe sleep sites do not sleep much and sleep very lightly.

In addition to mating and migration, sleep can also be reduced during food shortages, presumably to allow animals to invest more time in searching for the available food. Species whose environment has a severe seasonal variation in food availability have evolved to increase sleep during periods of food shortage and decrease sleep when food is available. (more)

GD Star Rating
Tagged as: , ,
Trackback URL:
  • ShardPhoenix

    I’ve been reluctant to try modafinil because of the perception that there might be unknown long-term health risks to skipping sleep, but this information has me updating in favour of it being a good idea.

    I’m still somewhat concerned about long-term effects of the specific chemical mechanism though.

    • dmytryl

      Look at the actual data on dolphins before you do so. Wikipedia is a fine starting point:

      The Indus river dolphin
      has a sleep method that is different from that of other dolphin
      species. Living in water with strong currents and potentially dangerous
      floating debris, it must swim continuously to avoid injury. As a result,
      this species sleeps in very short bursts which last between 4 and
      60 seconds.

      Frankly, the article is complete crap. There is no other way to describe it. The author gone over everything cherry picking just what’s consistent with his pet theory and discarding anything that immediately falsifies it. Falling for this and mistaking cherry picked selection of data consistent with a hypothesis for data actually supporting a hypothesis would make a fine example of bias to discuss.

      On the ‘energy conservation’ in general, while the energy use may be reduced a little during sleep, energy intake is zero, and for a plenty of animals it makes absolutely no sense to enter a slightly less active phase, for a short time, for very small reduction in consumed energy but very big reduction in energy intake. Some sleep appears to be absolutely essential; probably the algorithms by which brain works requires some off-line processing, such as synaptic scaling:

      What happens in homo sapiens upon sleep deprivation is not mere tiredness. It’s microsleeps and pieces of brain going inactive for small timespans. Too much impairment for something that could as well have been an urge similar to sex drive, if it was non-essential.

      • Nikki

        It’s almost like you are arguing that a Science article is wrong because Wikipedia says so.

        Oh wait… You are.

      • Michael Vassar

        It’s almost like you’re asserting that place of publication takes precedence over content of argument while assuming that all Science articles are in agreement with one another…

      • Nikki

        @ Michael Vassar

        I am indeed asserting that place of publication determines whether you should pay attention, if that is what you mean. Disagreement expressed on a website where anybody can publish whatever they please tells you nothing about the accuracy of content of a peer-reviewed journal.

        As to other Science articles, I made no assumptions at all. If you would like to cite contradicting information from the same or another trustworthy source, that will be a useful contribution. “Let’s ignore this article because maybe there was or will be another reliable one that disagrees with it” is not a valid argument.

      • Stephen R. Diamond

        I am indeed asserting that place of publication determines whether you should pay attention, if that is what you mean.

        Dmitryl said Wikipedia was a good place to start, not that it is the arbiter of fact, and he gave specific arguments against the Science article’s conclusions. There are no facts in contention here, to justify the degree of authority you assign to source.

      • dmytryl

        Wikipedia contains a lot of references to peer reviewed research, which you can click and read through (I said, starting point). In this particular case, there’s an utterly overwhelming body of facts that square very poorly with the pet theory in question. The article you are speaking of, does nothing of this kind – it is a cherry picked selection. The wikipedia on the other hand is not written by some sleep justification conspiracy. It references peer reviewed research without massive cherry picking. For this reason, wikipedia is generally an excellent starting point, especially on topics as common and non-obscure as sleep.

        When we see a huge selection of facts that do not contradict someone’s pet theory, and assume some good faith, we assume that author has not found contradictory facts. However I am beginning to get a feeling that nowadays you have to assume less and less good faith.

      • John Maxwell IV

        Place of publication *does* matter in this case, contra Michael Vassar, ’cause the argument is over whether the data is cherry-picked, and the source could be an indicator of this.  It’s not obvious to me that either a Science article or a Wikipedia article would be more likely to have cherry-picked data though.

      • dmytryl

        John Maxwell IV : one cherry picked yet solidly verified fact can falsify a theory, but a thousand cherry picked facts that agree with a theory don’t confirm it…

        Yes, there is huge variation to the length of sleep. There is also huge variation to the duration of the copulation even though the copulation is absolutely essential for sexually reproducing species.

        Drawing the conclusion that sleep is for energy conservation is silly. If sleep initially evolves for maintenance, energy conservation during sleep would evolve; if sleep initially evolves for energy conservation, off-line during-sleep maintenance would evolve. Sleep is evolutionarily very very old.

        Ohh, other terrible bit in the article is the very approach of thinking of evolution as having intent – implicit in the question “what is the sleep for?”. The animal sleep is consistent with both energy conservation and maintenance taking place; furthermore the conclusion is inescapable that some of said maintenance doesn’t evolve to happen during active state (and when it has to, parts of neural network shut down for maintenance). One could be very sceptical of maintenance if we couldn’t imagine why a data storage and retrieval system (let alone a half chemical one!) could possibly need to undergo periodic off-line processing. Alas, the real-time, on-line processing poses great difficulty to us in our intelligent design of any systems that have similar functionality.

        edit: also, bottlenose dolphins are an example of animal that is comparably brainy to humans, but unable to figure a way to negotiate something with us. Or in simple terms, animal that is not living up to it’s brain size.

      • Carl Shulman

        Ioannidis says that the highest impact journals are wrong at a high rate, since they differentially publish the sexiest, most counterintuitive (and more likely false) findings and try to scoop everyone else.

        An individual article can advance an idiosyncratic thesis, while wikipedia is more conservative (as a result of the collaborative structure).

      • ShardPhoenix

         Ok, but I’m not saying I want to skip all sleep, just that sleeping less would be desirable to save time. If only some sleep is actually required for brain function, and the rest is merely to conserve energy, then it’s more likely to be safe to eliminate the excess.

      • dmytryl

        Energy conservation is not automatically useful… you may be consuming less energy, but you are not getting much done either, and you are far less effective at survival. Animals aren’t battery powered toys, they have work to do just as much as you do. If you got lost in the woods with bears and other huge predators (or worst of all with feral humans), would you seriously want to sleep more than in comfy bed and with only some economical competition?

  • Michael Bishop

    This is interesting and relevant…

  • Simon Lambert

    It might be possible to compute the transformation of a brain from a tired state to a rested state a lot faster than to actually emulate a brain undergoing sleep. In which case, ems will probably have this transformation computed efficiently and skip the sleep.

  • Stephen R. Diamond

    Whether sleep (or even human sleep) evolved to conserve energy probably has a different answer than whether individual differences in sleep times evolved for that reason–or the converse.

  • dmytryl

    Regarding the EMs sleeping, I’d expect that the EM clock time would roughly correspond to number of neuron firings (and to the extent to which the firings are asynchronous), and so the sleep would run considerably faster by default and for everyone. Any maintenance which is bottlenecked by chemistry, or which is done slowly for sake of chemical efficiency, can run faster in brain emulator.

    Unless we are speaking of some far future brain emulations that emulate chemistry atom by atom, there’s no reason to expect them to run at constant speedup factor regardless of what you’re doing. Many optimizations of the emulator are possible even with very little understanding of how brain works.

  • Drewfus

    According to neuroscientist Daniel Wolpert, all brains are for movement.
    Sleep radically minimizes movement. During REM sleep the brain is decoupled from the body so that non-vital movement is impossible. What does ‘brains are for movement’ and the affect of sleep on movement suggest, when considered together?

    Consider that if the brain is not moving the body in some way, it has no purpose. The sleeping state must as a consequence, atrophy the brain. “Use it or lose it”.

    Why does the hindbrain want to atrophy the ‘higher’ (more recently evolved) areas of the brain? Is the body (including the spinal column and hindbrain) in some sense at war with the mid/forebrain? Does the hindbrain see the mid/forebrain as a parasite, trying to takeover and control the body? Does the sleep-wake cycle indicate a sort of tug-of-war between the body and brain that oscillates between alternating dominance? Does dreaming indicate a brain that is trying to regain control of its disconnected input-output mechanisms by creating virtual reality scenarios with its otherwise unused processing capacity? Does the mid/forebrain retaliate to this state of helplessness by terrorizing the hindbrain with nightmares?

    Some forebrains see our future as distinct from the body. The brain wants to rid itself of the body – eventually. Likewise the body with respect to the brain. That is, it wants to put the brain to sleep, in the same sense that vets put terminally sick animals to sleep.

    • Drewfus

      Something else that radically minimizes movement is being in the womb for nine months. How would the brain get stimulation when the body is stuck in a sack of warm, salty water for all that time? By dreaming?

      I think the real idea with sleep is to allow the brain to “drop” weak connections. Todays events might otherwise have too much influence on long-term memory. Current behaviour versus long-term learnt behaviour might have very different neuronal growth and pruning optimums, so perhaps when awake we operate in mode-A, but there is no mode-B – instead we do the cognitive version of fasting, and allow weak long-term connections built in the present day to fade. A big workaround. Perhaps if you have a traumatic event, you should sleep for a week?

      Btw, a nice doco about the first nine months and it’s health outcomes –

  • Stephen R. Diamond

    Is the apparently common phenomenon of sleep walking some evidence against the hypothesis that human sleep’s primary function is to save energy by immobilizing the organism.

  • mrwiizrd

    In Dragons of Eden, Carl Sagan posits that sleep may have evolved in our nocturnal mammalian ancestors in part as away for them to stay quiet and out of sight of reptilian predators on the hunt by day.  Not sure how plausible it is, but it’s an interesting premise.