Ecological Niche May Dictate Sleep Habits
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.