why the elderly have trouble sleeping?

As adults get older, they have trouble falling asleep and staying asleep. Symptoms of insomnia are especially pronounced among adults diagnosed with Alzheimer's disease. Now, researchers think they know why.
As part of a recent study, a group neurologists have pinpointed specific inhibitory neurons related to sleep. The researchers were able to show that the loss of these neurons resulted in increased sleep disruption.
Researchers were aided by a study on aging and dementia that began in 1997 and followed nearly 1,000 subjects from the age of 65 until to their deaths. Upon passing away, participants' brains were donated for research. Beginning in 2005, many of the study's participants agreed to wear digitized wristbands that recorded movement in 15 second increments as a way to document sleep habits.
Neuroscientists at the Beth Israel Deaconess Medical Center and the University of Toronto's Sunnybrook Health Sciences Center were able to examine the brains of 45 of these subjects and compare their findings to their sleep records in their waning years.
"We found that in the older patients who did not have Alzheimer's disease, the number of ventrolateral preoptic neurons correlated inversely with the amount of sleep fragmentation," explained Dr. Clifford B. Saper, a neurologist at Harvard Medical School and senior author of the new study. "The fewer the neurons, the more fragmented the sleep became."
"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," added Saper. "The loss of these neurons with aging and with Alzheimer's disease may be an important reason why older individuals often face sleep disruptions. 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."
The study was published this week in the latest edition of the journal Brain.