Study Reveals How Sleep
Helps Brain Development

WASHINGTON (Reuters) - Sleep, whose need is not questioned but whose function is shrouded in mystery, has a critical role in early brain development, scientists said on Wednesday.

Researchers at the University of California at San Francisco found sleep dramatically enhances changes in brain connections during a vital period of visual development in young cats.

They examined the effect of sleep on the brain after cats experienced an environmental challenge--vision blocked in one eye for six hours. They found that cats allowed to sleep for six hours after the experience developed twice the amount of brain change as those kept awake during that period.

The study appeared in the journal Neuron.

UCSF researcher Marcos Frank noted that every animal sleeps--even flies experience a sleep-like state. But despite major progress in the understanding of the neurobiology of sleep and the consequences of sleep deprivation, Frank said the reason the brain needs sleep has remained a mystery.

Michael Stryker, who heads the university's department of physiology, said the study may shed some light on the matter.

``One of the aspects of the mystery is that young animals and human babies sleep a whole lot more than they do when they get older,'' Stryker said in an interview. ``And this is precisely the time in life when the connections in the brain are being reorganized to attain the perfect precision that they have in normal adults.''

The capacity for brain change--growth and strengthening of connections between nerve cells--is the basis of early development in the brain, the researchers said. The enhancement and refinement of neural circuitry continues to a lesser extent in the adult brain, they added.

The process of growth, known as plasticity, is believed to underlie the brain's capacity to control behavior, including learning and memory. Plasticity occurs when neurons are stimulated by events, or information, from the environment.

Strong Evidence On Function Of Sleep

The study represented strong evidence that one function of sleep is to help consolidate the effects of waking experience on cortical plasticity, putting memories into permanent storage, Stryker said. While the study examined sleep's effect on young cats, sleep may play a similar role in older animals and people, he added.

``There may be similar phenomena going on in other areas of the brain later in life,'' Stryker said. ``I think it's likely to be true that other areas of the brain, higher areas of the brain, have their critical (developmental) periods later in life--and some of them, in the highest areas, the critical periods never close until senility.''

The study found the amount of plasticity in the brain depended on the amount of sleep known as non-rapid eye movement--the deep, quiet sleep marked by large, slow brain waves. This type of sleep alternates with periods of rapid eye movement, or so-called ``dream'' sleep, which is marked by rapidly changing brain waves and bursts of eye movement.

The researchers measured in cats the response of neurons of the brain's visual cortex to the six hours of blocked vision. They said the visual deprivation triggered a rapid remodeling of neural circuitry known as ocular dominance plasticity.

The researchers then examined the impact of sleep on the long-term effects of those changes by using brain imaging and making electrical recordings from brain cells.

The researchers determined that animals allowed to sleep for six hours after the period of visual deprivation developed twice the amount of brain change as those cats kept awake in a dark room during those six hours. The cats permitted to sleep also experienced twice the amount of change as those evaluated immediately following the period of visual deprivation.

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