October 27, 2005
Thursday

All animals need sleep, but scientists are increasingly discovering that neither snooze time nor type of sleep is anything close to uniform across species.

An analysis of research published Thursday concludes that animals, including humans, have adapted to sleep habits that restrict waking hours to times when they are most likely to be successful finding food and least likely to face danger.

"Conventional wisdom in much of neuroscience has been that sleep has a single vital function across animals, just as food and water have universal functions,'' said Jerome Siegel, chief of neurobiology research at the VA Greater Los Angeles Healthcare System and a professor of psychiatry at the University of California-Los Angeles.

"Yet some animals can go without sleep for long periods of time with no ill-effects, whereas sleep deprivation in others can be lethal,'' he said.

Siegel, a pioneer in translating animal sleep research into treating sleep problems in humans, wrote the analysis as part of a special section on sleep studies in the journal Nature.

Paradoxically, Siegel also suggests that sleep schedules are dictated by the brain metabolism rate of an animal - smaller animals tend to have higher rates of brain metabolism and tend to need more sleep to help the brain recover from some of the toxic side effects of energy production, while larger animals tend to have reduced sleep needs.

Thus, the tiny brown bat sleeps for about 20 hours each day, tree shrews sack out for nearly 16 hours, but large grazing land animals with slower metabolism - like the elephant, cow, giraffe and horse - get by on two to four hours of sleep. They need to stay awake to eat the rest of the time.

Meat-eating predators like tigers, lions, jaguars and domestic dogs and cats spend between two-thirds and half of each day asleep, a situation that Siegel and other researchers think may be both to conserve energy and help regulate body temperature.

Common in all mammals is some degree of rapid-eye-movement sleep, the deep, immobilizing sleep state that produces dreams in humans and perhaps animals. They all display the same sort of brain activity and increased heart rate variability during REM sleep. Most dog owners have seen their pets "chasing rabbits" during REM sleep, with the animals uttering soft woofs and twitching their legs.

On the other hand, Siegel noted animals that sleep relatively little, like cows and horses, get only short snatches of REM sleep during each day. "Animals with low sleep amounts do not seem to `make up' for low sleep quantity by sleeping more deeply when they do sleep,'' he explained.

One theory about REM sleep is that it serves to stimulate certain slumbering regions of the brain in advance of actually waking up - nature's way of making sure the brain' s not dangerously sluggish when consciousness returns.

Marine mammals, such as whales and dolphins, sleep with just half of their brains, and since they're constantly at some stage of alert that allows them to keep swimming and coming to the surface to breathe, they too experience little or no REM sleep.

Human sleep follows the same sort of environmental and eating-habit patterns that guide other omnivores, Siegel said, although people average about seven or eight hours of sleep a night, compared to 10 to 12 hours for other primates like baboons and rhesus monkeys.

Siegel speculates that humans have evolved to have more waking hours in order to "better compete with other humans." Studies have shown that people deprived of REM sleep due to certain psychoactive drugs or brain lesions don't seem to suffer any physical or cognitive consequences when they're awake.

On the Net:

www.nature.com

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