More than 18 million American adults have sleep apnea. The condition is more common in the elderly, especially those with Alzheimer’s disease. 

Poor sleep is linked to a wide range of possible ailments. However, it also works the other way around. Poor sleep can also be a sign that illness is contributing to the bad sleep.

Could a good night's sleep be a key to reducing the risk of developing Alzheimer's disease? It's still speculation, but a link between sleep and the rise and fall of a marker for the disease have given scientists hope.