Insomnia and fragmented sleep is a widespread problem among the elderly. However, until recently, scientists didn't know what caused it. Now, a group of researchers have discovered that a group of neurons that switches the brain 'off' for sleep could be lacking as we get older.

The study, done by researchers at Beth Israel Deaconess Medical Center and the University of Toronto/Sunnybrook Health Sciences Center, explains why older people get less quality sleep.

"On average, a person in his 70s has about one hour less sleep per night than a person in his 20s," says senior study author Clifford B. Saper, MD, PhD.

Sleep loss in the elderly is often associated with health issues like those occurring with cognitive decline, heart disease and diabetes. However, this new research shows another factor at play: the loss of a specific group of neurons.

In 1996, the research team discovered that a group of inhibitory neurons, called ventrolateral preoptic nucleus, was acting as a sort of switch in rats. This switch basically turns off the brain for sleep and then turns it back on after waking.  Scientists realized human brains acted similarly.

In those experiments, rats showed signs of insomnia when there was a loss of these neurons, sleeping about 50 percent less than their unaffected peers. And when they were asleep, their sleep wasn't productive: it was most often fragmented.

In their new study, researchers looked at data collected from a community initiative called the Rush Memory and Aging project. This project, which started in 1997, followed nearly 1,000 healthy 65-year-olds until death. After they passed away, they donated their brains for further research.

The researchers chose 45 of those subjects for their study. While they lived, these volunteers occasionally wore a device on their wrist that measured movement in sleep, monitoring how well they slept. Once scientists examined their brains, they discovered that those who had less inhibitory neurons were the most likely to suffer from insomnia and disrupted sleep patterns.

Scientists also discovered that the sleep problems were more pronounced in patients diagnosed with Alzheimer's, who showed greater losses of those neurons.

"These findings provide the first evidence that the ventrolateral preoptic nucleus in humans probably plays a key role in causing sleep, and functions in a similar way to other species that have been studied," says Saper. "These results may, therefore, lead to new methods to diminish sleep problems in the elderly and prevent sleep-deprivation-related cognitive decline in people with dementia."

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