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.”
Copyright 2005 The New York Times Company. Reprinted with permission.