Everyone knows that babies sleep a lot, but not always when Mom and Dad like.
Now researchers in San Francisco, who used young cats for a test, have found evidence that getting a lot of sleep early in life plays a crucial role in the brain's development.
While the new study focused specifically on the impact of sleep on brains tied to visual development in young cats, the scientists say their findings have broad implications both for development of brain structures in young animals and for the rewiring of neuronal connections.
"This is the first direct evidence that sleep modifies the effect of environmental stimuli on the development of new brain connections," said Marcos Frank, a postdoctoral fellow at the University of California-San Francisco and lead author of the report in the journal Neuron.
The research adds to evidence from other studies that sleep is required for the brain to maintain and strengthen neural networks.
"Every animal sleeps - even flies may have a state like sleep ... but why the brain needs sleep has remained a mystery," Frank said.
Younger birds and mammals, including humans, sleep up to three times as much as adults.
The research team, led by Michael Stryker, chairman of the university's physiology department, sought to measure the effect of sleep on the brain "plasticity" of cats after they had experienced an environmental challenge - having vision blocked in one eye for six hours. The team used brain imaging and recordings of electrical activity in brain cells to measure the changes.
Brain plasticity occurs when brain connections grow and are strengthened when neurons are stimulated by events or information from surroundings.
The visual deprivation caused a rapid change of brain circuitry connected to processing visual images. The team determined, though, that animals allowed to sleep for six hours after visual deprivation developed twice the amount of brain change as cats kept awake in a dark room during a similar period.
Specifically, the researchers noted that brain re-wiring most depended on how much non-rapid eye movement sleep the animals got. This type of deep, quiet slumber is marked large, slow brain waves, the kind of sleep people typically fall into when they first go to sleep. These quiet periods alternate with periods of so-called dream sleep, episodes of rapidly changing brain waves and rapid eye movement (REM).
Stryker said the findings lend credence to the theory that sleeping helps to consolidate waking experiences into the cortex of the brain, converting short-term memory into more permanent or enhanced forms. This seems to apply to both babies and older brains.
"The fact that sleep provoked slightly more plasticity than double the amount of exposure to experience (when the cats were kept awake) suggests that if you reviewed your notes thoroughly until you were tired and then slept, you'd achieve as much plasticity or learning in the brain as if you'd pulled an all-nighter," Stryker said.
A number of other researchers have shown that animals and humans deprived of sleep don't do as well on memory tasks, but also indicate that getting sleep after learning a new task or subject is crucial to retaining it over the long haul.
Most recently, scientists at Harvard Medical School reported in November that people taught a new task retained it better when allowed to sleep that night compared with another test group denied sleep.
"We think that getting that first night's sleep starts the process of memory consolidation," said Robert Stickgold, an assistant professor of psychiatry who led the study. "It seems that memories normally wash out of the brain unless some process nails them down. My suspicion is that sleep is one of those things that does the nailing down."
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