"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 brainthe cortexin 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 midbrainan 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 sleepdata 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 adultthe 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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