Infant, Adult Sleep Similar, Rat Study Says

Brian Handwerk
for National Geographic News

April 19, 2005

Adults spend nearly a third of their lives asleep. Infants sleep twice that much or even more—yet scientists don't completely understand the processes of sleep and why the state is so essential.

"We do know a lot today about the importance of sleep and the consequences of not getting enough sleep—or enough quality sleep," said Carl E. Hunt, director of the National Institutes of Health's National Center on Sleep Disorders Research in Bethesda, Maryland. "What we don't know is why the body was designed that way. We don't know why sleep is so critical. Clearly we know it's a fundamental requirement of the brain in order to continue to function at its best—yet we don't know why," Hunt said.

But new research on rats has added a piece to this puzzle by revealing that infant and adult sleep are strikingly similar.

Mark Blumberg, of the Department of Psychology at the University of Iowa, and colleagues have reported that the neural (nerve) mechanisms in the active sleep of week-old rats are similar to the sleep mechanisms of adult rats.

The results suggest that the basic components of sleep are present soon after birth and develop in more or less a straight line throughout life.

"Fundamentally there are many more similarities than differences between rat and human sleep. Most of the basic phenomena—physical twitching, changes in cortical activity, REM movements, and many others are similar. Infant rats are also similar to human babies in the way that they rapidly cycle in and out of sleep. In both species, these cycles become longer as we age," Blumberg said.

"Every parent is aware of their infant's tendencies to rapidly go back and forth between sleep and wakefulness or to sleep all day. That's what we see in rats. So why does sleep change developmentally and become more scheduled and regulated so that we sleep through the night? What's changing here? [We're trying] to understand the basic features of the brain that regulate sleep, and then understand how sleeping cycles change with age," he said.

Led by fellow University of Iowa researcher Karl Karlsson, Blumberg and colleagues published their findings this week in the journal PLoS Biology.

Tracking Brain Changes

In 1953 University of Chicago researchers discovered rapid eye movement (REM) sleep, the sleep most closely associated with dreaming. Before then, the conventional wisdom was that the brain simply "shut down" during sleep.

In fact, while the body rests, the brain itself cycles through periods of rest and activity. "You have intriguing changes going on in the brain [during sleep], and we don't know what they are for, what they represent functionally," Blumberg explained.

Tracking these neural functions in infants proved a challenge.

"The key to establishing that the brain is involved in infant sleep is demonstrating changes in brain activity as it cycles in and out of sleep," Blumberg said.

But some measurements of surface brain activity, like EEG (electroencephalogram), don't show much activity in the surface layer of the brain—the cortex—in very young mammals. As a result, some researchers have questioned whether infant and adult sleep are fundamentally different.

"If cortical activity is considered a key feature for demonstrating adult sleep but this activity doesn't occur in infants, then can we say that infants sleep?" Blumberg asked, evoking the old philosophical saw about whether a chair missing a leg is still a chair.

But the team was able to measure infant brain activity using a variety of techniques.

It was a challenge working with tiny animals that alternately sleep and wake at 10- to 15-second intervals. "First, we used a tracing technique to see which parts of the brain would light up," Blumberg said.

They found crucial activity in the midbrain—an area known to be important for adult sleep. Next, recording electrodes were placed in these regions to determine if they contain neurons that exhibit activity specific to sleep and wakefulness.

"Some neurons became active when [an infant rat] went to sleep, some when it woke up. Some neurons became active when the animal was twitching, so they are not only sleep-active but [specific to] REM sleep," Blumberg said.

The team was able to further pinpoint neural functions by creating lesions in specific parts of the brain. These procedures induced some rats to sleep as much as 95 percent of the time, or prevented others from sleeping to the same degree.

All told, the data revealed important similarities between infant and adult sleep—data that the scientists say are applicable to humans as well as rats.

"These data fill a gap in our knowledge and help to better understand the continuum of sleep development as we go from infant to child, adolescent, and adult—the full age spectrum," Hunt, of the National Institutes of Health, said. "This provides some insight into how [this process] is indeed a continuum."

Understanding such sleep processes may be an important step in more precisely determining the role sleep plays for both infants and adults.

More than 40 years ago researchers Howard Roffwarg and William Dement found that babies spent much more time in REM sleep than adults. They hypothesized that infant REM sleep might help develop the central nervous system for the sensory experiences of adulthood.

The current results might aid in testing this intriguing theory and other theories that relate to infant sleep.

"We sleep much more in infancy, and I think [this research] helps us to justify increasing our focus on infant sleep," Blumberg said. "Theories of sleep that are irrelevant to infants are unlikely to have global importance for our understanding of sleep."

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