Neurodegenerative diseases have been often associated with sleep disorders, mainly when they are characterized by abnormal accumulation of alpha-synuclein, such as multiple system atrophy (MSA), Parkinson's disease (PD) and Lewy body disease (LBD).[21][22][23][24][25][26][27] For instance, people diagnosed with PD have often presented different kinds of sleep concerns, commonly regard to insomnia (around 70% of the PD population), hypersomnia (more than 50% of the PD population), and REM sleep behavior disorder (RBD) - that may affect around 40% of the PD population and it is associated with increased motor symptoms.[21][22][23][24][25][27] Importantly, RBD has been also highlighted as a strong precursor of developing dementia in PD patients over several years in prior, which seems to be a great opportunity for improving the treatments of the disease.[22][24]
The neurodegenerative conditions are commonly related to brain structures impairment, which might disrupt the states of sleep and wakefulness, circadian rhythm, motor or non motor functioning.[22][24] On the other hand, sleep disturbances are also frequently related to worsening patient's cognitive functioning, emotional state and quality of life.[24][27][28] Furthermore, these abnormal behavioural symptoms negatively contribute to overwhelming their relatives and caregivers.[24][27][28] Therefore, a deeper understanding of the relationship between sleep disorders and neurodegenerative diseases seems to be extremely important, mainly considering the limited research related to it and the increasing expectancy of life.[22][28]
^ Hirshkowitz, Max (2004). "Chapter 10, Neuropsychiatric Aspects of Sleep and Sleep Disorders (pp 315-340)" (Google Books preview includes entire chapter 10). In Stuart C. Yudofsky; Robert E. Hales (eds.). Essentials of neuropsychiatry and clinical neurosciences (4 ed.). Arlington, Virginia, USA: American Psychiatric Publishing. ISBN 978-1-58562-005-0. ...insomnia is a symptom. It is neither a disease nor a specific condition. (from p. 322)
In Alzheimer's disease, in addition to cognitive decline and memory impairment, there is also significant sleep disturbances with a modified sleep architecture.[30][29] The latter may consist in sleep fragmentation, a reduction in sleep duration, insomnia, an increase daytime naping, a decreased quantity og some sleep stages and a resemblance between some sleep stages (N1 and N2).[29] More than 65% of people with Alzheimer's disease suffer from this type of sleep disturbance.[29]
Throughout the decades as more research and studies were conducted, the amount of sleep disorders being discovered began to rapidly increase. In 1990 The AASM, along with other professional societies including the European Sleep Research Society, The Japanese Society of Sleep Research, and the Latin American Sleep Society published the International Classification of Sleep Disorders (ICSD), which is a "primary diagnostic, epidemiological, and coding resource for clinicians and researchers in the field of sleep and sleep medicine."
Sleep apnea, obstructive sleep apnea, obstruction of the airway during sleep, causing lack of sufficient deep sleep, often accompanied by snoring. Other forms of sleep apnea are less common.[10] Obstructive sleep apnea (OSA) is a medical disorder that is caused by repetitive collapse of the upper airway (back of the throat) during sleep. For the purposes of sleep studies, episodes of full upper airway collapse for at least ten seconds are called apneas[11]

At the same time, it has been shown that memory consolidation in long-term memory (which depends on the hippocampus) occurs during NREM sleep.[29][33] This indicates that a decrease in the NREM sleep will result in less consolidation and therefore poorer memory performances in hippocampal-dependent long-term memory.[29][33] This drop in performance is one of the central symptoms of AD.[29]  
A catch-all term for the inability to fall or stay asleep, sleeplessness is -- as sufferers know -- a very serious problem. It's sleeplessness whenever something, whether it's physical pain, anxiety, or an underlying condition, prevents you from falling asleep within a reasonable amount of time or staying asleep long enough to achieve a good night's sleep.
Another systematic review noted 7-16% of young adults suffer from delayed sleep phase disorder. This disorder reaches peak prevalence when people are in their 20s.[57] Between 20 and 26% of adolescents report a sleep onset latency of >30 minutes. Also, 7-36% have difficulty initiating sleep.[59] Asian teens tend to have a higher prevalence of all of these adverse sleep outcomes than their North American and European counterparts.[59]
The natural internal clock that controls our 24-hour cycle of sleep and waking, circadian rhythms are easily upset by changes in schedule, and they're greatly affected by light and darkness. Jet lag is the best known circadian rhythm disorder, but this sensitive inner clock can also be disrupted by changes in routine resulting in an erratic sleep schedule.
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