Tuesday, November 08, 2005

critter sleep

Down for the Count
By CARL ZIMMER, The New York Times, November 8, 2005

In a laboratory at Indiana State University, a dozen green iguanas sprawl tranquilly in terrariums. They while away the hours basking under their heat lamps, and at night they close both eyes - or sometimes just one. They lead comfortable lives pretty much indistinguishable from any ordinary pet iguana, except for one notable exception: the bundles of brain-wave recording wires that trail from their heads.

A team of scientists at Indiana State would like to know what happens in the brains of the iguanas when the lights go out. Do they sleep as we do? Do they shut the whole brain down, for example, or can they keep one half awake?

These scientists in Terre Haute hope the iguanas will also help shed some light on an even more fundamental question: why sleep even exists.

"Sleep has attracted a tremendous amount of attention in science, but we really don't know what sleep is," said Steven Lima, a biologist at Indiana State.

Dr. Lima belongs to a small but growing group of scientists who are pushing sleep research deep into the animal kingdom. They suspect that most animal species need to sleep, suggesting that human slumber has an evolutionary history reaching back over half a billion years.

Today animals sleep in many different ways: brown bats for 20 hours a day, for example, and giraffes for less than 2. To understand why people sleep the way they do, scientists need an explanation powerful enough to encompass the millions of other species that sleep as well.

"One of the reasons we don't understand sleep is that we haven't taken this evolutionary perspective on it," Dr. Lima said.

Sleep was once considered unique to vertebrates, but in recent years scientists have found that invertebrates likes honeybees and crayfish sleep, as well. The most extensive work has been carried out on fruit flies. "They rest for 10 hours a night, and if you keep them awake longer, they need to sleep more," said Dr. Giulio Tononi, a psychiatrist at the University of Wisconsin.

The parallels between fruit flies and humans extend even to their neurons. The two species produce, during part of the night, low-frequency electrical activity known as slow-wave sleep. "The flies surprised us with how close they were in many ways," Dr. Tononi said.

Discovering sleep in vertebrates and invertebrates alike has led scientists to conclude that it emerged very early in animal evolution - perhaps 600 million years ago. "What we're doing in sleeping is a very old evolutionary phenomenon," Dr. Lima said.

Scientists have offered a number of ideas about the primordial function of sleep. Dr. Tononi believes that it originally evolved as a way to allow neurons to recover from a hard day of learning. "When you're awake you learn all the time, whether you know it or not," he said.

Learning strengthens some connections between neurons, known as synapses, and even forms new synapses. These synapses demand a lot of extra energy, though. "That means that at the end of the day, you have a brain that costs you more energy," Dr. Tononi said. "That's where sleep would kick in."

He argues that slow waves weaken synapses through the night. "If everything gets weaker, you still keep your memories, but overall the strength goes down," he said. "The next morning you gain in terms of energy and performance."

Dr. Tononi and his University of Wisconsin colleague, Dr. Chiara Cirelli, present this hypothesis in a paper to be published in the journal Sleep Medicine Reviews. Dr. Tononi believes it can be tested in the future, as scientists document sleep in other animal species. "It would be a very basic thing that would apply to any brain that can change," he said.

It has been almost 600 million years since human ancestors diverged from those of flies. As those ancestors evolved, their sleep evolved as well. Human sleep, for example, features not only slow-wave sleep, but bouts of sleep when the eyes make rapid movements and when we dream. Rapid eye movement, or REM sleep, as it is known, generally comes later in the night, after periods of intense slow-wave sleep.

Other mammals also experience a mix of REM and non-REM sleep, as do birds. Sleep researchers would like to know whether this pattern existed in the common ancestors of birds and mammals, reptilian animals that lived 310 million years ago. It is also possible that birds and mammals independently evolved this sleep pattern, just as birds and bats independently evolved wings.

Answering that question may help scientists understand why REM sleep exists. Scientists have long debated its function, suggesting that it may play important roles in memory or learning. In the Oct. 27 issue of Nature, Jerome Siegel, a sleep expert at the University of California, Los Angeles, argues that REM does not play a vital physiological role like slow-wave sleep. He points out that brain injuries and even medications like antidepressants can drastically reduce REM without any apparent ill effect.

"People who don't have REM sleep are remarkably normal," Dr. Siegel said. "There's no evidence for any intellectual or emotional problems."

So why do mammals and birds have REM sleep at all? "The best answer I can come up with is that it's there to prepare you for waking," Dr. Siegel said. "When the important work of sleep is done, REM sleep just makes you as alert as you can be while you're asleep."

One advantage to being alert but immobile is that you may be better able to escape a predator. Dr. Lima and his colleagues argue in the October issue of Animal Behavior that sleep may have been profoundly shaped during evolution by the constant threat of predators. From this perspective, it is strange that animals would spend hours each day in such a vulnerable state. "It's so stinking dangerous to be shut down like that," Dr. Lima said.

It is possible to imagine an alternative way to let the brain recover: only put small parts of the brain to sleep at a time. But Dr. Lima and his colleagues present a mathematical model suggesting that shutting down the whole brain at once may actually be safer.

"You may be better off just shutting down and sleeping all at once, and do it quickly," Dr. Lima said. "Even though you're fairly vulnerable while you're asleep, your overall vulnerability in a 24-hour period may be lower."

Birds appear to be able to defend against predators with a variation on this strategy. When they feel safe, they sleep with their entire brains shut down, as humans do. But when they sense threats, they keep half their brains awake.

Dr. Lima and his colleagues have demonstrated this strategy in action with several bird species, including ducks. "All we did was put our ducks in a row, quite literally," said Niels Rattenborg, a colleague of Dr. Lima's, now at the Max Planck Institute for Ornithology in Germany. "The ducks on the interior slept more with both eyes closed, and the ducks on the edge slept with one eye open. And they used the eye that was facing away from the other birds."

To give each side of the brain enough rest, the ducks at the ends of the row would stand up from time to time, turn around and sit down again. This allowed them to switch eyes and let the waking half of the brain go to sleep.

The Indiana State team is now studying iguanas to see if they sleep with half their brains, as well. Previous studies have shown that lizards keep one eye closed for long periods of time, but it has not been clear if they have also been half asleep. Monitoring iguana brains with electrodes may give the scientists an answer.

If reptiles and birds turn out to sleep this way, it may be evidence that it is an ancient strategy. It is even possible that the earliest mammals also slept with half a brain. "It's possible that early on in mammal evolution they may have lost it for some reason," Dr. Rattenborg speculated. "It may have conflicted with other functions."

On the other hand, some species of whales and seals sometimes swim with one eye closed while the corresponding hemisphere of the brain produces slow waves. Scientists are still debating whether they are actually asleep in this state. If they are, that suggests that the ancestors of marine mammals reinvented half-brain sleeping. It may have re-emerged as an adaptation to life in the ocean, an environment where predators can come out of nowhere.

While humans and other land mammals may not be able to shut down half the brain, they may be able to cope with predators by adjusting their sleep schedules. Some studies on rats suggest that predators cause the animals to cut back on slow-wave sleep. People often react to stress in the same way.

"Some of the changes we observe in people who are experiencing stress may be some of the same mechanisms in response to predators," Dr. Rattenborg said. "There are no lions sneaking up on them, but the daily stresses of our lives may activate this primordial response."

Dr. Tononi believes that studying animals may ultimately help doctors find more effective ways to treat such sleep disorders. "There are no good guidelines about what is satisfactory sleep, because there is no idea of what it does," he said. "Is seven hours of very light sleep O.K.? Or is deep sleep very important, or REM?"

He added: "It might really be that you can do with less sleep as long as it's doing its job. That's why it's crucial to know what its job is."

Monday, November 07, 2005

mammalian sleep

Clues to the functions of mammalian sleep
Jerome M. Siegel1,Nature 437, 1264-1271 (27 October 2005) doi: 10.1038/nature04285

Top of pageAbstractThe functions of mammalian sleep remain unclear. Most theories suggest a role for non-rapid eye movement (NREM) sleep in energy conservation and in nervous system recuperation. Theories of REM sleep have suggested a role for this state in periodic brain activation during sleep, in localized recuperative processes and in emotional regulation. Across mammals, the amount and nature of sleep are correlated with age, body size and ecological variables, such as whether the animals live in a terrestrial or an aquatic environment, their diet and the safety of their sleeping site. Sleep may be an efficient time for the completion of a number of functions, but variations in sleep expression indicate that these functions may differ across species.

Saying that it is desirable to be well rested and that the body seeks lost sleep with a vigour comparable to or greater than that displayed for food or sex does not answer the question of the functional role of sleep. Why do we spend one-third of our lives asleep? Why has our body evolved to press us relentlessly to make up for lost sleep? Can we separate the drive for sleep, manifested in sleepiness, from the function of sleep, as we can separate hunger from the benefits of food consumption? Why do so many species habitually sleep much more than humans, and others much less, and how do species that sleep for only short periods accomplish the functions of sleep in less time? Why does the daily sleep amount decrease from birth to maturity in all species of terrestrial mammals? And why do we have two kinds of sleep, rapid eye movement (REM) and non-REM (NREM) sleep?

Sleep can be defined as a state of immobility with greatly reduced responsiveness, which can be distinguished from coma or anaesthesia by its rapid reversibility. An additional defining characteristic of sleep is that when it is prevented, the body tries to recover the lost amount. The existence of sleep 'rebound' after deprivation1 demonstrates that sleep is not simply a period of reduced activity or alertness regulated by circadian or ultradian rhythms, a phenomenon that can be seen even in non-sleeping organisms2, 3, 4.

Read the rest here.

Sunday, November 06, 2005

circadiana commentary

(Non) Adaptive Function of Sleep

Here is a nice article in Washington Post - Ecological Niche May Dictate Sleep Habits - about the adaptive function of sleep.

It addresses some of the themes I am interested in.

First, the unfortunate fact is that sleep was initially defined by researchers of humans, i.e., medical researchers. Inevitably, the (electrophysiological) definition of sleep was thus saddled with unneccessary anthropocentric elements that for decades hampered the study of evolution of sleep.

Read the rest here.

Saturday, November 05, 2005

sleep by niche

Ecological Niche May Dictate Sleep Habits
By Shankar Vedantam, Washington Post Staff Writer, Monday, October 31, 2005

For centuries, poets, philosophers and scientists have debated why humans spend as much as a third of their lives asleep.

For Shakespeare, sleep was the "balm of hurt minds" -- denied to murderers such as Macbeth. For Sigmund Freud, sleep provided a platform for dreams, an outlet for the psyche to work out complex and dangerous feelings. Scientists today believe sleep consolidates learning and memory, and supports many essential mental and physical functions.

The theorists have long disagreed about one another's ideas, but most agree on one thing: If nature makes people sleep away so much of their lives, the reason has to be something crucial. That seemed to be the only way to explain why sleep-deprived people crave sleep so badly that they doze off behind the wheel of a car going 60 mph, and why rats deprived of sleep die sooner than rats deprived of food.

Yet a wealth of sleep research has regularly produced baffling paradoxes and conflicting lines of evidence about the uses, role and need for sleep. If sleep is primarily about providing mental rest, why do people's brains remain so active during sleep, as research in recent decades has found?

If sleep is about providing the body with rest, why do couch potatoes need as much sleep as Olympic athletes? Moreover, animals such as horses, which perform far more physical labor than humans, need much less sleep than people do.

If sleep primarily hones cognitive functions, why do the intellectually lazy need as much sleep as Nobel Prize-winning physicists? Also, why do humans -- who are a lot smarter than rats -- sleep less than rodents?

Finally, while much conventional thinking suggests that Americans should be sleeping more, a very large 2002 study found that people who sleep eight hours or more a night are likely to die younger than those who sleep seven. (Don't touch that alarm clock; the study did not find that deliberately sleeping less increases life span.)

Jerome Siegel, a psychiatrist at the University of California at Los Angeles who described these discordant findings in acomprehensive review of the available research, published in the journal Nature last week, said he began to question the notion that sleep performs some essential function after noting that species that sleep less than others do not sleep any deeper -- as they would if they were making up for the shorter time. Animals that sleep fewer hours generally sleep less deeply, while animals that sleep longer usually sleep more deeply.

Siegel, a respected sleep researcher who is also affiliated with the Department of Veterans Affairs, said he came to the conclusion there was only one explanation that could explain the paradoxes: in a word, evolution.

Rather than being designed to perform some critical function, Siegel wrote in his paper, sleep may be the way various species, humans included, have adapted to their ecological niches. While many valuable functions probably take place during sleep, Siegel suggested that it is possible that those functions are not the reason for sleep.

"There is this huge variation in sleep across species, and it fits with this huge variation in the niches that animals occupy," Siegel said in an interview.

"The analogy I make is between hibernation and sleep," he said. "No one says, 'What is hibernation for? It is a great mystery.' . . . It's obvious that animals hibernate because there is no food, and by shutting down the brain and body they save energy."

Sleep, Siegel suggested, may play much the same role. As evidence, he cited research that has found systematic differences in the way carnivores, omnivores and herbivores sleep: Carnivores sleep longer; herbivores, shorter; and omnivores, including humans, are somewhere in the middle.

"If animals have to eat grass all day, they can't sleep a lot, but if they eat meat and are successful at killing an antelope, why bother to stay awake?" he asked.

On the other hand, mammals at greater risk of being eaten -- such as newborns -- spend large amounts of time asleep, presumably safe in hiding places devised by their parents. Supporting the evolutionary explanation, Siegel's own research has shown that when the luxury of safe hiding places is unavailable -- in the ocean, for instance -- baby dolphins and baby killer whales reverse the pattern found among terrestrial mammals. These marine mammals sleep little or never as newborns and gradually increase the amount they sleep as they mature.

The theory does not so much contradict other theories about the role of sleep as much as place them in context: "What I am saying is that it is not that sleep has been adapted to allow some vital function to be fulfilled, but the core function of sleep is to adapt animals to their ecological niche," Siegel said. "Given the animal is inactive for a certain period of the day, certain functions will migrate to that period because it is more efficient" to perform them at that time.

Mark Mahowald, a sleep researcher at the University of Minnesota who wrote about sleep disorders in the same Nature issue, agreed that Siegel's work had shown that "sleep, which likely serves vital functions, serves different functions across the various species."

But Clifford Saper, a neuroscientist at Harvard Medical School and an author of another sleep-related article in last week's Nature, about circadian rhythms, said that despite the variations in sleep across species, "the universality of sleep in all creatures with nervous systems suggests a basic principle that requires explanation."

Saper pointed to the work of his Harvard colleague Robert Stickgold, who published a fourth article in Nature last week describing how memory is consolidated during sleep. Saper said sleep provides brain cells with a period of "down time" that is needed to convert information into learning.

That change, he said, involves biochemical messages being sent from nerve cell projections called dendrites to the nucleus of cells, and that process can take place only when new information is not coming in. The variety of sleeping styles among species, Saper suggested, may be merely linked to their different cognitive needs.

"What a fruit fly learns and what a whale learns and what a human learns may be very different indeed, and so the amount of sleep, and its structure within a day, may be different in each species," he said.