The Influence of Insomnia on Developing Neurodegenerative Disorders

A 66 year old man came to the neurology clinic with complains of “slowness” and gradually progressive memory loss over the last 2 years. On examination, he was found to have slow finger tapping in both hands, right sided upper limb rigidity and a shuffling gait. He said that he has been working as an entrepreneur for the last 30 years and he has been working 16-18 hours every day and he described his work as “highly stressful”. He also said that he has always been a “short sleeper” with an average of only 4-5 hours of sleep every night. He has no family history of neurodegenerative disorders.

The annual financial burden of the two most common neurodegenerative diseases in the U.S.; Alzheimer’s disease (AD) and Parkinson’s disease (PD) is about 277 billion and 25 billion respectively (1, 2). The number of deaths from of Alzheimer Disease and other dementia continue to rise every year despite advances in medical sciences probably due to an aging population and with no definitive treatment or cure available to patients (2). Therefore, it would be logical to study the risk factors leading to neurodegenerative diseases until some form of treatment is discovered. One of the most important and changeable risk factors leading to dementia later in life is poor sleep (3).

Insomnia is defined when a person has hard time falling asleep or maintaining sleep, waking up earlier than expected or having non-restorative sleep, while short sleeper is defined as a person who can only sleep less than 7 hours on a nightly basis (4, 5). Non-restorative sleep, as the name suggests refers to the sleep that does not restore the body despite having normal sleep period, efficiency and quality (6). Insomnia, whether primary or secondary, is frequently associated with neurodegenerative diseases. It has been found that insomnia and AD have a two-way relationship with AD causing sleep fragmentation and poor sleep leading to increased deposition of beta-amyloid and hyper-phosphorylated Tau protein in human brain and subsequent AD (3). It has also been described that insomnia in old age leads to cellular aging (7). Chronic insomnia also indirectly increases the risk of dementia through increasing the risk of depression, hypertension, diabetes and obesity (8, 9, 10, 11, 12). But much less is known if chronic insomnia especially non - restorative sleep and chronic short sleepers increase the risk of AD and other neurodegenerative diseases. It is also yet to be discovered if sleep problems in childhood like primary insomnia and insomnia secondary to parasomnia or sleep phase delay syndrome could lead to neurodegeneration later in life.

In this review article, we will try to summarize the data available from previous studies and prove if there is a link between insomnia especially non-restorative sleep, chronic short sleep, primary and secondary insomnia in childhood and neurodegenerative disorders. We will also summarize the polysomnography findings available in previous research studies that could support the association between insomnia and neurodegeneration. If insomnia is identified as an individual risk factor for neurodegenerative disorders, a new door of preventative and treatment strategies will open that could totally change the incidence and course of neurodegenerative disorders.