Sleep disorder
A sleep disorder, or somnipathy, is a medical disorder that disrupts an individual's sleep patterns and quality. This can cause serious health issues and affect physical, mental, and emotional well-being. Polysomnography and actigraphy are tests commonly ordered for diagnosing sleep disorders.
Sleep disorders are broadly classified into dyssomnias, parasomnias, circadian rhythm sleep disorders, and other disorders. When a person struggles to fall or stay asleep without an obvious cause, it is referred to as insomnia, which is the most common sleep disorder. Other sleep disorders include sleep apnea, narcolepsy, hypersomnia, sleeping sickness, sleepwalking, and night terrors.
Sleep disruptions can be caused by various issues, including teeth grinding and night terrors. Managing sleep disturbances that are secondary to mental, medical, or substance abuse disorders should focus on addressing the underlying condition.
Sleep disorders are common in both children and adults. However, there is a significant lack of awareness about sleep disorders in children, with many cases remaining unidentified. Several common factors involved in the onset of a sleep disorder include increased medication use, age-related changes in circadian rhythms, environmental changes, lifestyle changes, existing physiological problems, and stress. Among the elderly, the risk of developing sleep-disordered breathing, periodic limb movements, restless legs syndrome, REM sleep behavior disorders, insomnia, and circadian rhythm disturbances is especially high.
Causes
A systematic review found that traumatic childhood experiences, such as family conflict or sexual trauma, significantly increase the risk of several sleep disorders in adulthood, including sleep apnea, narcolepsy, and insomnia.An evidence-based synopsis suggests that idiopathic REM sleep behavior disorder may have a hereditary component. A total of 632 participants, half with idiopathic REM sleep behavior disorder and half without, completed self-report questionnaires. The study results suggest that people with the sleep disorder are more likely to report having a first-degree relative with the same sleep disorder than people of the same age and sex who do not have the disorder. More research is needed to further understand the hereditary basis of sleep disorders.
A population susceptible to the development of sleep disorders includes people who have experienced a traumatic brain injury. Due to the significant research focus on this issue, a systematic review was conducted to synthesize the findings. The results indicate that individuals who have experienced a traumatic brain injury are disproportionately at risk of developing narcolepsy, obstructive sleep apnea, excessive daytime sleepiness, and insomnia.
Obstructive sleep apnea is a common condition affecting 10-20% of middle-aged and older adults, characterized by repeated breathing pauses during sleep, leading to poor sleep quality, and excessive daytime somnolence and, sometimes insomnia. Common factors include obesity, narrow airways, and certain neuromuscular conditions that cause airway collapse during sleep.
Sleep disorders and neurodegenerative diseases
Neurodegenerative diseases are often associated with sleep disorders, particularly when characterized by the abnormal accumulation of alpha-synuclein, as seen in multiple system atrophy, Parkinson's disease, and Lewy body disease. For example, individuals diagnosed with PD frequently experience various sleep issues, such as insomnia, hypersomnia, and REM sleep behavior disorder , which is linked to increased motor symptoms. Moreover, RBD has been identified as a significant precursor to the future development of these neurodegenerative diseases over several years, presenting a promising opportunity for improving treatments.Neurodegenerative conditions are commonly related to structural brain impairments, which may disrupt sleep and wakefulness, circadian rhythm, and motor or non-motor functioning. Conversely, sleep disturbances are often linked to worsening of patients' cognitive functioning, emotional state, and quality of life. Additionally, these abnormal behavioral symptoms can place a significant burden on patients' relatives and caregivers. The limited research in this area, coupled with increasing life expectancy, highlights the need for a deeper understanding of the relationship between sleep disorders and neurodegenerative diseases.
Sleep disturbances and Alzheimer's disease
Sleep disturbances have also been observed in Alzheimer's disease, affecting about 45% of the affected population. When based on caregiver reports, this percentage increases to about 70%. As in the PD population, insomnia and hypersomnia are frequently recognized in AD patients. These disturbances have been associated with the accumulation of beta-amyloid, circadian rhythm sleep disorders, and melatonin alterations. Additionally, changes in sleep architecture are observed in AD. Although sleep architecture seems to naturally change with age, its development appears aggravated in AD patients. Slow-wave sleep potentially decreases, sleep spindles and the length of time spent in REM sleep are also reduced, while REM latency increases. Poor sleep onset in AD has been associated with dream-related hallucinations, increased restlessness, wandering, and agitation related to sundowning—a typical chronobiological phenomenon in the disease.In Alzheimer's disease, in addition to cognitive decline and memory impairment, there are also significant sleep disturbances with modified sleep architecture. These disturbances may consist of sleep fragmentation, reduced sleep duration, insomnia, increased daytime napping, decreased quantity of some sleep stages, and a growing resemblance between some sleep stages. More than 65% of people with Alzheimer's disease experience this type of sleep disturbance.
One factor that could explain this change in sleep architecture is a disruption in the circadian rhythm, which regulates sleep. This disruption can lead to sleep disturbances. Some studies show that people with Alzheimer's disease have a delayed circadian rhythm, whereas in normal aging, an advanced circadian rhythm is present.
In addition to these psychological symptoms, there are two main neurological features of Alzheimer's disease:
- The first is the accumulation of beta-amyloid waste, forming aggregate amyloid plaques.
- The second is the accumulation of tau protein.
On the other hand, it is during sleep that beta-amyloid residues are degraded to prevent plaque formation. The glymphatic system is responsible for this through the phenomenon of glymphatic clearance. Thus, during wakefulness, the beta-amyloid burden is greater because metabolic activity and oxidative stress are higher, and there is no protein degradation by glymphatic clearance. During sleep, the burden is reduced as there is less metabolic activity and oxidative stress, in addition to the glymphatic clearance that occurs.
Glymphatic clearance occurs during NREM SWS sleep, a stage that decreases with normal aging, leading to reduced glymphatic clearance and increased beta-amyloid burden, which forms plaques. Therefore, sleep disturbances in individuals with Alzheimer's disease may exacerbate this phenomenon.
The decrease in the quantity and quality of NREM SWS, along with sleep disturbances, will therefore increase the Aβ plaques. This initially occurs in the hippocampus, a brain structure integral to long-term memory formation. As hippocampal cell death occurs, it contributes to reduced memory performance and cognitive decline found in AD.
Although the causal relationship is unclear, the development of AD correlates with the onset of prominent sleep disorders. Similarly, sleep disorders exacerbate disease progression, forming a positive feedback loop. As a result, sleep disturbances are not only a symptom of AD; the relationship between sleep disturbances and AD is likely bidirectional.
At the same time, it has been shown that memory consolidation in long-term memory, which depends on the hippocampus, occurs during NREM sleep. This indicates that a decrease in NREM sleep will result in less consolidation, leading to poorer memory performance in hippocampal-dependent long-term memory. This drop in performance is one of the central symptoms of AD.
Recent studies have also linked sleep disturbances, neurogenesis, and AD. The subgranular zone and subventricular zone continue to produce new neurons in adult brains. These new cells are then incorporated into neuronal circuits in the subgranular zone, which is found in the hippocampus. These new cells contribute to learning and memory, playing an essential role in hippocampal-dependent memory.
However, recent studies have shown that several factors can interrupt neurogenesis, including stress and prolonged sleep deprivation. The sleep disturbances encountered in AD could therefore suppress neurogenesis and impair hippocampal functions. This suppression would contribute to diminished memory performance and the progression of AD, while the progression of AD would further aggravate sleep disturbances.
Changes in sleep architecture in patients with AD occur during the preclinical phase of the disease. These changes could potentially be used to detect those most at risk of developing AD. However, this is still only theoretical.
While the exact mechanisms and causal relationship between sleep disturbances and AD remain unclear, these findings provide a better understanding and offer possibilities to improve the targeting of at-risk populations, as well as the implementation of treatments to curb the cognitive decline of AD patients.