The older you get, the less you sleep. This statement has a lot of evidence behind it, but the reason why has been pretty unknown.

A new study, published in Brain, offers, for the first time, a reason as to why this is the case: a certain cluster of neurons called the ventrolateral preoptic nucleus may die off as people get older. The study was done by researchers at the Beth Israel Deaconess Medical Center and the University of Toronto.

"The more of these cells you lose from aging, the harder time you have sleeping," said Dr. Clifford Saper, lead researcher for the study, told Huffington Post.

He said older people are sleeping less and then getting up because they can't sleep anymore, even if they still feel tired. Sleep fragmentation can lead to many issues such as cognitive dysfunction, high blood pressure and the tendency to develop type 2 diabetes.

This study could pave the way for more research and possible medication to smooth the disrupted sleep patterns without negatively affecting other brain functions such as balance.

Saper found the first association between the neurons and sleep disruption in rats. He found that rats that lacked this bundle of neurons suffered from insomnia.

Saper then tried to replicate the findings in a data set of nearly 1,000 human subjects. The study began in 1997 with a group of healthy 65 year-olds who had to wear a watch-sized device on their wrist all day for 7 to 10 days, every two years. After they died, their brains were donated to science.

Saper examined 45 of the brains and looked at the ventrolateral preoptic nucleus - whether it was still intact and how intact it was. He stained the brain to find the cluster of neurons. He compared the neuron clusters to data about sleep fragmentation during that person's last year of life. He found that the fewer neurons a person had in that area, the more fragmented their sleeping patterns were.

Another key finding Saper found was that the link between the neurons and sleep fragmentation was more pronounced in people with Alzheimer's disease.

Saper said this finding could help researchers understand Alzheimer's patients better and possibly find a way for them to stay at home with their families rather than being institutionalized.

"If we could develop medications that helped Alzheimer's patients sleep through the night, without causing them to have increased risk of falling, this could keep some people out of nursing homes," he said.