Then they observed the same participants over a three-night study period, during which the researchers deliberately disturbed their sleep when their brain waves indicated the beginning of slow wave sleep.
The sounds used to interrupt the sleep patterns were loud enough to move the participants to a different level of sleep but not loud enough to fully wake them. According to the researchers, the participants could recall hearing between three and 15 noises at night, although they were interrupted on average 250 to 300 times. The interruptions increased in number each night, as the participants' need for deep sleep increased.
"This decrease in slow-wave sleep resembles the changes in sleep patterns caused by 40 years of aging," Tasali said. Young adults spend 80 minutes to 100 minutes per night in slow-wave sleep, while people over age 60 generally have less than 20 minutes. "In this experiment," she said, "we gave people in their 20s the sleep of those in their 60s."
At the end of each study, the researchers gave intravenous glucose (a sugar solution) to each subject, then took blood samples every few minutes to measure the levels of glucose and insulin, the hormone that controls glucose uptake.
When the researchers analyzed the data they learned that the participants were almost 25 percent less sensitive to insulin after nights of interrupted sleep. As their insulin sensitivity declined, they needed to make more insulin to process the same amount of glucose, or blood sugar. However, in all but one subject, their bodies did not make more insulin. As a result, they had 23 percent more blood-glucose, the equivalent of glucose levels in an older adult with impaired glucose tolerance.
The researchers also found that the participants who typically had the least amount of slow-wave sleep during the nights they were not interrupted experienced the greatest decline in insulin sensitivity du
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