Biology concepts – theories of sleep, REM sleep, circadian rhythms, neural plasticity
You open the door to your house and find your roommate sprawled out on the couch. Is he sleeping, unconscious, or dead? Knowing your roommate, you figure it could be any of them – you stop yourself short of naming a preference.
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You find your roommate passed out in his underwear, and can’t decide if he is sleeping, unconscious or dead. If you have chosen Homer as a roommate, you have already clued us in to your decision-making abilities. |
The live/dead question is easy; hold a mirror under his nose and see if it fogs up. If he’s not breathing, there’s only one thing to do – go through his pockets and look for loose change (with a nod to “The Princess Bride”). But if you do see condensation, how do you decide if he is passed out or just sleeping - or are you considered unconscious when sleeping?
Sleep is voluntary, at least most of the time. I try to stay awake at the ballet, but I don’t always succeed. But besides drinking yourself into a stupor, going unconscious is usually not voluntary. Unfortunately (or fortunately), you weren’t there to see what preceded the crease marks on your roommate’s face or his drooling on the couch pillow, so how can you identify his state?
Sleeping implies that one has a diminished ability to respond to external stimuli with reduced sensory perception. However, unconsciousness appears much the same. The difference lies in the degree of diminished capacity; you can be roused more easily from sleep and perhaps not at all from deeper unconsciousness. Some people I know must pass out every night, because they are tough to wake up. You might parse the difference and just say that sleep is more easily rousable unconsciousness.
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Sleep has stages and these stages have cycles. If deprived of a particular stage the night before, your body will change your cycles so that you make up the lost time in that stage on the next night. Source for image: http://xavier appsychology.wikispaces.com/Chapter+5,+ Period+6 |
In general, the safer an animal is, the more it dreams. Predators dream more than prey and big species dream more than small species, though there are several exceptions to this rule. For example, ruminants (cows, deer, goats, and buffalo) dream very little (about 5 minutes/night), and cetaceans (whales, dolphins, porpoises) may not dream at all.
In contrast, animals that are born immature (not able to live on their own) tend to experience lots of REM sleep. These altricial (meaning “requiring nourishment") animals, including marsupials, cats, dogs, and most rodents, may have 6-8 hours of REM sleep a night. What is more, as adults they continue to dream heavily – about what, I have no idea.
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Do you know the differences between dolphins and porpoises? Dolphins have longer bodies and snouts, and porpoises have a straight front edge on their dorsal fin. But, they are both cetaceans and have the same sleep patterns. Opposums, the only marsupials in North America, have immature young, and for some reason they dream much more. They are probably dreaming about the day their kids will get off their back. |
But even this exception has an exception. Many birds are born very immature. They have no feathers, they can’t fly, they usually have their huge eyes closed, so they are definitely altricial species. But, birds have extremely short cycles of non-REM and REM sleep. Avian REM cycles might total only 5 minutes in a night, and each episode might be only 9 seconds long. What can you get done in a 9 second dream?
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Brain waves recorded on an electroencephalogram (EEG) show that dolphins and birds have normal activity in one hemisphere while the other is at rest. The heartbeat is constant showing that there is normal body rhythm. This is unihemispheric sleep. Image is taken from: Ridgeway, S. et. al. J. Exp. Med. 209:2902-2910, 2006. |
REM sleep is deeper and harder to be roused from compared to non-REM sleep, the short cycles might be related to birds’ sleep pattern, which is unihemispheric (one half of the brain) in non-REM sleep, and is probably related to their need to keep watch for predators. Birds don’t lose muscle tone when they sleep; often they have to remain on a perch while they sleep. How embarrassing it would be for a bird to fall asleep and then fall off their branch- they would deserve to be eaten.
Other species of bird can sleep while flying, the arctic tern for example, whose migration can be as long as 22,000 miles one way. During flight, the eye connected the active half of the brain will remain open to navigate, but the bird will not dream, since both hemispheres are required in all animals for REM sleep.
Dreaming less doesn’t necessarily correlate with sleeping less. Animals that dream little may still sleep a considerable amount. For prey animals, sleep may represent a dicey time when they must be on the lookout. But it might also represent a way to stay motionless, blend in, and avoid predators. Either theory is practical, since predators seem to take the old, young, and diseased, whether sleeping or not.
Indisputably, every animal needs to sleep to survive, but why? It is interesting that science hasn’t quite figured this out yet. It is known that many beneficial events occur during sleep, but just being good for you doesn’t make them vital. But it must be vital, since even hibernating animals will cycle from hibernation to sleep in order to reap the benefits. Several theories exist for the necessity of sleep:
Energy conservation theory of sleep. Smaller animals carry less fat than large animals, which means they have a smaller margin of error in energy usage – they must conserve energy or feed more often. By sleeping longer, smaller animals keep their metabolic rate low and conserve more energy for when they need it, like for finding more food.
Related to this, animals with fewer predators seem to sleep longer than animals who may be hunted by many other species (as discussed above). However, since resting saves 90% as much energy as sleeping and that animals could watch for predators while resting, there clearly must be additional reasons to sleep instead of just rest.
Repair theory of sleep. This theory contends that non-REM sleep is important for repairing the physical body. Indeed, cell division and protein synthesis increase during non-REM sleep. On the other hand, REM sleep is necessary for restoring mental function, but we will leave the reasons for why we dream for another discussion.
Information packaging theory of sleep. You may sleep in order to provide the brain with time to process all that occurred the previous day, and be ready to take in more the next day. This relates to something called neural plasticity (new connections, ie. learning) and memory consolidation. Recent evidence shows that sleep deprivation harms recall, so sleep may help move information from short-term to long-term memory.
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One theory of sleep is that your brain returns to a set point so you can learn things the next day. Learning means making new connections between neurons; these connections are reinforced by neurotransmitters being released to stimulate the next neuron in series. If the neuron isn’t fired, it will not release neurotransmitters to stimulate the next neuron in the path. If they are not repeatedly fired, the pathway will no longer exist, and new connections can be made. |
In terms of plasticity, a 2007 study indicated that the slow brain waves in non-REM sleep are linked to our ability to learn new information. Dr. Guioli Tononi stated that neural connections become progressively weaker during slow wave sleep, so that by morning, the connections are ready to record new information, but still strong enough to hold the old memories.
An extension of this study, published in 2011 by the same group showed that in some groups of neurons, synapse size and number was affected by the amount of sleep that fly and the amount of experience that the fly had. More experience required more sleep in order to prune the connections and strengthen those that were used repeatedly. After a few hours of wake, synapse size and number increased, and sleep was required to reduce those that are weak and strengthen the remaining circuits.
An extension of this study, published in 2011 by the same group showed that in some groups of neurons, synapse size and number was affected by the amount of sleep that fly and the amount of experience that the fly had. More experience required more sleep in order to prune the connections and strengthen those that were used repeatedly. After a few hours of wake, synapse size and number increased, and sleep was required to reduce those that are weak and strengthen the remaining circuits.
If sleep provides all these benefits, and higher animals can’t survive without it (even insects and worms have periods of inactivity that look a lot like sleep), then how is it that the giraffe sleeps only 2-4 hours per day? As prey, nature may have deemed it more important to stay alert; or maybe they just can’t find a long enough blanket.
Cetaceans, like birds, only let half their brain sleep at a time, so they probably don’t dream either. Being mammals, they still have to be able to surface to inhale and exhale while sleeping, called conscious breathing. This might require that some part of their brain be active at all times.
I say might because most cetacean sleep studies have been done in captive animals (the smaller species). But in 2008, the boat of a cetacean research team accidentally floated into the middle of a pod of inactive sperm whales. The whales were unresponsive to the researchers and had both eyes closed. This agreed with another observation that electronically tagged sperm whales spent about 7% (1.68 hr/day) of their drifting with the tide. If this is true sleep (not unihemispheric), it would be a new finding in cetaceans and would indicate that that sperm whales sleep less than any other mammal.
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The American bullfrog is fully alert when inactive, so is it asleep? Scientists think that the bullfrog is so territorial and is such a good parent that it will not let its guard down until it dies. |
An even more amazing exception to the sleep rule is the American bullfrog (Rana catesbeiana). Brain wave studies (electroencephalography) of the nocturnally active bullfrog did show signs of rest during the day, but bullfrogs had no loss of sensory perception. They could react to stimuli just as if awake. Other frogs show similar brain waves, but are much harder to arouse. The bullfrog might be the only animal to pull a lifetime all-nighter. He should really be ready for that math test.
Nobody yet knows exactly how sleep restores the brain or why bullfrogs and giraffes need so little, but we do know that people who are deprived of sleep suffer physically, emotionally, and intellectually – or worse. How would you like to be condemned to death for not taking a nap? We’ll talk about this next time.
For more information, classroom activities, and laboratories on theories of sleep, and sleep in animals, see:
sleep –
theories of sleep – http://psychology.about.com/od/statesofconsciousness/p/TheoriesofSleep.htm
stages of sleep and REM sleep –
sleep in animals –
http://thebrain.mcgill.ca/flash/capsules/outil_jaune07.html
