Sleeping on the Wing By Robin Krieger Mejia Published in the Winter 1998 issue of Exploring Magazine

Feeding is not too difficult; the birds are well adapted to making quick dives for ocean fish.  But they cannot spend very much time in the ocean without becoming waterlogged.  Terns spend a lot of time flying over ocean waters, but are not really adapted to floating.  One would imagine, however, that the tern may hit points in its commute when it needs to sleep, but there is no suitable perch in sight.  Evolution has provided an answer to this problem:   the tern is able to put one half of its brain to sleep while the other half remains alert and functioning.  Talk about running at half capacity -- terns can actually function with only half a brain awake, and the sleep they get this way seems to meet their needs.  Scientists call this unihemispheric sleep, because the tern alternately lets the left and the right hemisphere of its brain sleep. Other animals share this type of sleep pattern; it is common in several bird species and dolphins.

The idea of unihemispheric sleep almost begs a definition of sleep itself, it certainly appears to have little to do with what we do at night -- drifting off into complete unconsciousness.  In reality, sleep is hard to define in terms of perception, partially because it is always difficult for us to guess how an animal feels, or perceives its environment.  However, scientists have developed measurements of brain electrical activity that occur during sleep and have defined several types of sleep according to the types of electrical signals the brain is generating.  This allows them to determine experimentally if an animal is sleeping, or just resting, by monitoring its brain wave activity.  This has traditionally been done in the laboratory, but improved equipment is facilitating the move to studying animals in their natural environments.

One thing we have learned from this type of study is that sleep is actually an active process -- the brain generates different types of measurable brain waves that are not present when an animal is awake.  In humans and most animals, the entire brain does this in concert, and the animal becomes unconscious as part of the process, but in a few animals, one half, or hemisphere of the brain can become unconscious and at a time, while the other half remains conscious to the outside world.

Unihemispheric sleep  is only one example of the many intriguing sleep adaptations in the animal kingdom.  Many animals exhibit sleep patterns remarkably different, in their pattern, quality, duration, and even existence than our own.  However, all mammals that have been studied, and most terrestrial vertebrates, do sleep -- even if sleep appears to put them at risk to some danger, or cause another difficulty, such as the tern's migration problem.  The fact that no higher animals have been able to evolve out of the need for sleep, combined with the fact that need for sleep correlates with the development of advanced brains, has lead researchers to the conclusion that sleep must serve some necessary function in most vertebrates.

There is some debate in the scientific community as to what are the reasons that animals sleep; however, one theory that is widely accepted is that sleep gives the brain "down time" to recover from the wear and tear it receives while awake. However, there is some debate in the scientific community as to other functions of sleep, as it appears to provide different benefits to different species. In his book "Why We Sleep: The Functions of Sleep in Humans and Other Mammals," author James Horne points out that, on top of giving the brain down time, sleep may serve different functions in different organisms including helping the animal conserve energy, occupying an animal during monotonous periods where it cannot feed, or during periods where venturing away from the nest may be dangerous.

Small rodents, for example, are unable to rest while they are awake -- they genuinely lack the ability to stay still or contemplate anything.  And, the smaller an animal is, the higher its metabolism, so rodents use up a lot of energy during their waking hours.  Not surprisingly, these animals often sleep over half of the day away.  For rodents, energy conservation may be an important reason why they  sleep as much as they do, as they save a great deal of energy by sleeping.  Also, because they cannot rest while awake sleep is the only time small rodents are still -- and stillness is necessary to allow the body and muscles time to recover after a full day of running around. Combine these benefits with the fact that  their small size allows them to retire to safety in hidden nests or burrows, and sleeping seems like a fairly good use for a good part of their day.

The situation of many animals is not so favorable to sleep. Sheep have been observed to spend only about four hours per day sleeping.  However, they also spend about four hours  per day in a state of drowsiness -- opening and closing the eyes, and not moving except for chewing their cud. This ability to rest without sleeping allows their bodies to perform some maintenance of muscles and tissues while they are awake.  Sheep are very vulnerable to attack while asleep, and it is likely that they evolved to sleep as little as possible while still maintaining whatever essential functions the body requires sleep for.  Most other rudiments, including cows, show similar patterns. Giraffes, observed in the wild, appear to sleep for only three to 75 minutes at a time, in short intervals spaced throughout the night.

So what is the essential function of sleep that makes animals do it, even when it poses a threat to their safety, or requires the evolution of unusual patterns such as putting one half of the brain to sleep at a time?  Horne notes that there is agreement that sleep facilitates some type of self maintenance by the brain, and research is starting to point to what that might be.

Sleep has evolved along with the development of more complex brains and improved senses in animals and, in particular, the move from the sea onto land.  Evolution has allowed animals to develop more sensitivity to their surroundings, through improved sight, hearing, touch, taste and smell, and the brain has developed to process all of this incoming stimuli.

J. L. Kavanau of the University of California, Los Angeles Department of Biology recently published a commentary in Neuroscience, in which he explained that there is growing evidence that an important function of sleep is to give the brain time to encode memories.  It appears that as animals and their brains evolved, the job of interpreting its immediate environment grew  so large that the brain had to evolve some sort of "down time" to allow for the encoding of memories and possibly other maintenance functions.

The idea is that in order to maintain memories, the brain has to periodically "exercise" the neural networks in which they are encoded. Many of our memories may not be called apon for years at a time, but the brain needs to make sure that they will be there when we need them.  You've hear the adage about never forgetting how to ride a bike -- well the information has to be stored somewhere, and presumably taken out and checked on once in a while.  Also, the process of organizing, prioritizing, and encoding memories from all of the input that the brain gets each day appears to require some of this down time.

Sleep can be divided into several components by scientists, but the largest distinction in made between rapid eye movement (REM) sleep, which is characterized by observable eye movements, and non-rapid eye movement (NREM) sleep. These types of sleep are also distinguishable by the electrical signals the brain produces.  According to Kavanau, mammals are the only animals that have evolved REM sleep, which is the part of sleep during which humans experience dreams.  All animals that sleep show NREM sleep.

During NREM sleep muscle tone relaxes in the body, and for ectothermic ("cold blooded") animals this relaxation provides protection against disruptive body movements during sleep that would interrupt brain maintenance processes.  This is important, because many memories include movement -- and it would disrupt sleep if the body tried to follow through with a kick when every time the brain got to a memory that included one.  But as mammals evolved higher body temperatures and metabolic rates, they probably became more likely to move during NREM sleep, which would  disrupt the encryption of memories involving movement.  During REM sleep, the brain disables the connection between the centers that generate the command for body movement and the muscles, which could allow the brain to process memories that involve movement.  This correlates with our experiences: dreams occur only during REM sleep, and most of us have dreams that include movement (who dreams of just lying still in bed every night?)  yet, in general most of us don't actually do what we dream about.

Unihemispheric sleep, like that of the tern, appears to consist only of NREM sleep, even though it occurs in dolphins and other sea mammals (which tend to have high metabolic rates.)  Scientist conjecture that because these animals are constantly moving, they are maintaining the viability of the circuits that encode for those commands.  Essentially, their constant repetitive swimming may take care of same function that the brain does during REM sleep in most animals.

As we noted earlier, however, animals have evolved to take advantages of other benefits of sleep, such an conservation of energy, and the immobility it provides to animals that cannot rest while awake.  Evolution has taken the conservation of energy one step further in some species; many small animals use sleep as an entry into a state called torpor, where they can lower their body temperature to near zero, stop most brain activity, and conserve even more energy.

Animals generally go into torpor during times of extreme low temperatures or a scarce food supply.  In torpor, the animal's heartbeat and breathing  slow and its metabolic rate decreases significantly and its brain patterns can vary between those of NREM sleep and virtually no activity at all -- which demonstrates that the benefits of energy conservation are probably the selective pressure that has allowed development of torpor.  During periods of no activity the brain is obviously not performing maintenance functions.   In fact, animal has to periodically "wake" itself to the level of consciousness of NREM and REM sleep in order to sustain torpor with muted brain activity.

The Arctic ground squirrel undergoes this type of deep hibernation.  While the squirrel's temperature normally hovers around 37° C, when it hibernates it can drop its temperature to between -2° and 5° C, reducing its metabolism to 1/8 of its waking rate.  At this temperature, the squirrel's brain appears inactive.   However, every one to three weeks, the squirrel spontaneously returns to a normal body temperature and spends about 18 hours there.  During this time the squirrel experiences NREM and REM sleep, and during the last 6 hours it wakes, although it remains curled in its hibernating posture.  After this warmth and dream break, the squirrel can reenter torpor for another one to three weeks.

Different animals experience different patterns of hibernation; those that hibernate at a higher temperature may remain in the realm of NREM sleep most of the time.  However, all hibernators undergo periodic warmings and, if they have been in deep torpor, a return to a measurable sleep state.

This is different than the winter sleep of bears, which is not hibernation at all, but something called seasonal sleep. Many bears can spend up to three months mostly asleep, which is an unusually long time, but they really just sleep. Their body temperature remains normal and they are easily woken up.  Females even suckle their young during this time. In fact, they probably do not save that much energy though the long sleep. Horne hypothesizes that the bear's seasonal sleep may have evolved simply to overcome the monotony of long, dark, winters. Kind of sound appealing with El Nino coming, doesn't it?

Elephant seals exhibit another sleep adaptation: controlled sleep apnea.  Apnea, or stopped breathing, occurs during the sleep of other mammals, but it is generally a harmful condition.  In humans, apnea causes fitful sleep, as the person's body repeatedly wakes itself when breathing stops.  Elephant seals, however, have evolved to survive without breathing for minutes on a regular basis.  A study of four month old elephant seal pups by researchers at the University of California, Santa Cruz showed regular periods of sleep apnea of up to 12 minutes separated by short periods of regular breathing.  The pups can even sleep with their head underwater; they raise their head to the surface for periodic breathing without waking up.  The adaptation of this sleeping pattern has little to do with brain maintenance.  Rather, it helps the seals conserve water.  Elephant seals have not been observed to drink during the three months they spend on land each year.  They rely entirely on water metabolized from fat, and the extended sleep apneas help them conserve this water (much water is lost during the breathing process.)  The seals are able to breath more efficiently than most mammals, obtaining more oxygen from each breath.  This capability most likely developed to facilitate the deep dives that the seals do while at sea, but has shown a secondary adaptability when combined with sleep, allowing them to stay beached longer during the year.

Other intriguing sleep patterns exist in the animal kingdom, probably including many that have not yet been noted or studied by scientists.  What this brief survey of different patterns shows us is that the concept of sleep covers many more types of activities than our human forty winks.  While sleep appears to be necessary across many species barriers, and probably serves a similar purpose of brain maintenance, nature has facilitated adaptations that allow sleep to fit into a the huge variety of lifestyles we see in the animal kingdom.
 

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