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Sleep disorders can be characterized as a disorder of sleep patterns. These can include disorders that disrupt sleep, make it difficult to fall asleep, or even disorders that cause abnormally long time periods of sleep. Disorders that cause involuntary sleep deprivation, or even voluntary sleep deprivation, are becoming of greater and greater interest as their negative effects on the brain and body are becoming widely accepted in the scientific community1. As an example, there have been many studies, one reported in Nature recently in 2009, which indicates that sleep deprivation has a negative impact on cAMP signalling in the hippocampus, synaptic plasticity and hippocampus dependent memory2. However, before addressing the effects of sleep deprivation, it is important to understand some of the disorders which cause it. Three of the most common sleep disorders which disrupt sleep patterns are Sleep Apnea, General Insomnia and Restless Leg Syndrome (RLS). Narcolepsy, which is basically the opposite of insomnia, is also of interest in studying the effects of sleep deprivation and it allows appreciation of the flip side; defects and damage that can be caused to the brain and body from too much sleep. Because sleep seems to be very important in a variety of cognitive functions, such as learning, memory consolidation and language development, which are thought to be at the center of what it is to be a complex being- sleep disorders and deprivation can have a major effect on individuals and their daily functioning. This page will focus on briefly giving an overview of generally accepted background information about each of these disorders, and then presenting current and ongoing research on each aspect.


Sleep Apnea

1.1 Brief Overview and Classification

Overview
Sleep apnea is characterized as irregular or intermissions in breathing during sleep. This can happen as a result of obstruction in the upper airways (Obstructive Sleep Apnea), or even as a result of lack of motivation (caused by an imbalance in respiratory centers in the brain) to breathe (Central Sleep Apnea). Every period of obstructed breathing is called an apnea. Every case is different and so these can occur anywhere between five and thirty times in an hour of sleep. Common symptoms and side effects of sleep apnea include insomnia, hypoxemia and snoring.

Obstructive Sleep Apnea
Sleep apnea is characterized as irregular or periods of lack of breathing during sleep. When this occurs as a result of obstruction in the upper airways it is referred to as Obstructive Sleep Apnea (OSA). OSA is the more common of the two main types of sleep apnea. In essence, low muscle tone and excess tissue around the airways can collapse during the night, obstructing the airways. This is from a combination of low muscle tone and lack of input from the motor neurons which dilate the upper airways during the night. This is commonly found in individuals with obesity. This can also be as a result of a respiratory infection however chronic cases are of more interest as they can have harsher implications on the individual.

Central Sleep Apnea
Central sleep apnea (CSA) is characterized by imbalances in the brain’s respiratory centers during sleep, results in periods of apnea. It is mainly attributed to abnormal feedback loops and inaccurate monitoring of blood carbon dioxide levels. Chemoreceptors do not detect the change in carbon dioxide levels and as a result the signal to inhale is not sent to the respiratory system. The individual does not even struggle or attempt to breathe. This can result in hypoxaemia (lack of oxygen) or hypercapnia (excess carbon dioxide). This can further have severe impacts on the body and brain, more severe outcomes occurring with longer periods of apnea, and mild, recoverable symptoms resulting from shorter periods. After a period of CSA, the individual may exhibit increased breathing in order to compensate.

1.2 Current Research


General Insomnia

2.1 Brief Overview

Insomnia is in its simplest of forms, the inability to fall or stay asleep. Many mental disorders and terminal illnesses, such as major depressive disorder and cancer exhibit insomnia as a major symptom; that is, insomnia is secondary or co-morbid with them. There are three major classifications of insomnia; transient, chronic and acute. Transient insomnia is that which lasts a short period of time (less than a week) and is usually secondary or co-morbid, that is, it is generally associated with another disorder. Second, chronic insomnia is long lasting (longer than a month), almost on a daily basis. Chronic insomnia can also be co-morbid however it also is a primary disorder a large proportion of the time. That is, it is the disorder not the symptom. Last, acute insomnia is insomnia which occurs for a short period of time (less than a month). Acute insomnia can be due to stress, environment changes, etc. It can be co-morbid, secondary or primary. A large part of insomnia is the associated lack of functioning as insomnia worsens; this is because sleep deprivation can have significant impacts on the body. All types of insomnia are also commonly associated with drug use and abuse.

2.2 Current Research


Restless Leg Syndrome

3.1 Brief Overview

Restless leg syndrome (RLG) is characterized by the uncontrollable need to move one’s body, in particular one’s legs. This is characterized as a neurological disorder however can also be considered a sleep disorder due to the nature of the episodes. Episodes of tingling, pain, urge to move one’s legs and restlessness most commonly occur at night thus interrupting or deterring sleep. In addition, laying down and relaxing generally aggravates the symptoms of RLG. Similarly to other sleep disorders, many of the effects of RLG are consistent with the effects of sleep deprivation as sleep deprivation largely results from RLG. However, characteristic of this disorder is the absence of symptoms in the early morning. Thus, individuals can sometimes recover lost sleep during this time. In severe cases of RLG, episodes occur twice a week. A less severe form of RLG is called periodic limb movement during sleep (PLMS). This is characterized by involuntary twitching and jerking during the night and can worsen and develop into RLG but that is not always the case.

3.2 Current Research


Narcolepsy

4.1 Brief Overview

Narcolepsy is, in its simplest of terms, the opposite of insomnia. It is characterized by excessive and inappropriate sleepiness and sleep attacks. Characteristic of narcoleptic patients is that they experience REM sleep within five minutes of falling asleep whereas most individuals do not until about an hour into sleep. Also, narcoleptic individuals sometimes experience difficulty falling asleep at night but excessive sleepiness in the day, thus sometimes their abnormal sleep patterns can be mistaken as insomnia. A common side effect of narcolepsy is known as cataplexy. Cataplexy is a sudden period of lack of muscle tone, usually brought on by emotion. In rare, severe cases of cataplexy, individuals may also experience paralysis.

4.2 Current Research


Sleep Deprivation

5.1 Neurological Effects

There is an array of cognitive effects that are easily identified after sleep deprivation. The visible, identifiable effects of sleep deprivation include attention lapses, loss of patience, inability to focus and impaired long term memory. Neurological effects can sometimes be more difficult to identify for the average person.

In 2000, a study, using fMRI technology, found that during a verbal learning task, regions of the prefrontal cortex and parietal lobe were activated in sleep deprived subjects but the temporal lobe was not. The opposite was seen in well rested subjects. This is an evident disorganization as the temporal lobe is associated with language processing but was not activated in sleep deprived patients.

An association has also been found between non-rapid eye movement sleep (NREM) link and inhibition of neurotransmitters. It is suggested that NREM sleep is a time during which neurotransmitters can retract, thus allowing receptors to regain sensitivity as well. As a result, there is evidence which suggests rapid eye movement sleep (REM) acts as a selective serotonin reuptake inhibitor (SSRI) and thus can have an impact on depression. This association of neurotransmitters with the various phases of sleep highlights more effects of sleep deprivation. If neurotransmitters are constantly active due to lack of sleep, naturally, this can decrease receptor sensitivity in the brain and consequently it can become more difficult to feel their effects. This is a significant reason why insomnia and depression have such high co-morbidity.

A study in 1999 found that an enhancement in the hypothalamic- pituitary- adrenal axis and inhibited growth hormones as a result of sleep deprivation.

Microsleeps are a manifestation of sleep deprivation. They result of excessive sleep deprivation and are essentially when the brain temporarily shuts down for a few seconds. Microsleeps are experienced as black outs as they are not voluntary or under the control of the individual.

5.2 Physiological & Metabolic Effects


Current Topics in the study of sleep deprivation



Gujar et al. (2011) have done extensive work on brain reward networks and the effect that sleep deprivation has on them. This is interesting because unlike many others, it reveals that sleep deprivation can in fact amplify reactivity of brain reward networks. This was shown using fMRI technology, displaying that the primary visual processing pathways and limbic system showed improved networks where medial frontal and orbitofrontal regions showed decreased networks. In addition, the mesolimbic reward pathways in the brain are stimulated more than expected when exposed to pleasurable stimuli. This is the complete opposite of what is claimed by most studies to date as insomnia (and narcolepsy) is usually attributed to depression and downregulation of reward pathways. As hopeful as this may sound for sufferers of sleep deprivation, this is still a negative impact as it shows that there are affective imbalances in the brain which may provide immediate gratification but can have harmful long term effects. In addition, due to the diversity between the way different individuals react to different disorders, this effect could have significantly different impacts on different individual.