|Classification and external resources|
Parasomnias are a category of sleep disorders that involve abnormal and unnatural movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or arousal from sleep. Most parasomnias are dissociated sleep states which are partial arousals during the transitions between wakefulness and NREM sleep, or wakefulness and REM sleep.
NREM parasomnias are arousal disorders that occur during stage 3 (or 4 by the R&K standardization) of NREM sleep—also known as slow wave sleep (SWS). They are caused by a physiological activation in which the patient’s brain exits from SWS and is caught in between a sleep and waking state. In particular, these disorders involve activation of the autonomic nervous system, motor system, or cognitive processes during sleep or sleep-wake transitions.
Some NREM parasomnias are common during childhood but decrease in frequency with increasing age (sleep-walking, night-terrors, and confusional arousal). They can be triggered in certain individuals by alcohol, sleep deprivation, physical activity, emotional stress, depression, medications, or a fevered illness. These disorders of arousal can range from confusional arousals, somnambulism, to night terrors. Other specific disorders include sleepeating, sleep sex, teeth grinding, rhythmic movement disorder, restless legs syndrome, and somniloquy.
With a prevalence of 4%, confusional arousals are not observed very often in adults; however, they are common in children. Confusional arousals are occasional thrashings or inconsolable crying among children—they are characterized by movements in bed.
Sleep terrors is the most disruptive arousal disorder since it may involve loud screams and panic; in extreme cases, it may result in bodily harm or property damage by running about or hitting walls. Unfortunately, all attempts to console the individual are futile and may prolong or intensify the victim’s confused state. Usually the victim experiences amnesia after the event but it may not be complete amnesia. Up to 3% of adults suffer from sleep terrors and exhibited behavior of this parasomnia can range from mild to extremely violent. 
Bruxism is a common sleep disorder where the sufferer grinds their teeth during sleep. This can cause sleep disruption for the sufferer and bed partner, wear and fracture of teeth, and jaw pain.
Restless leg syndrome sufferers report an itching, burning, or otherwise uncomfortable feeling in their legs, usually exacerbated when resting or asleep. This causes sleep disruption as they wake to move or scratch their legs.
Periodic limb movement disorder is a sudden involuntary movement of limbs. It can cause sleep disruption and injury for both the sufferer and bed partner. Many people who have restless leg syndrome also have periodic limb movement disorder.
REM Sleep Behavior Disorder is the most common REM sleep parasomnia in which muscle atonia is absent. This allows the individual to act out their dreams and may result in repeated injury-- bruises, lacerations and fractures-- to themselves or others. Patients may take self-protection measures by tethering themselves to bed, using pillow barricades or sleeping in an empty room on a mattress.  Demographically, 90% of RBD patients are males, and most are older than 50 years of age. 
Typical clinical features of REM sleep behaviour disorder are: - male gender predeliction - mean age of onset 50–65 years (range 20–80 years) - vocalisation, screaming, swearing that may be associated with dreams - motor activity; simple or complex that may result in injury to patient or bed-partner - occurrence usually in later half of sleep period (REM sleep) - may be associated with neurodegenerative disease 
Chronic RBD is idiopathic or associated with neurological disorders. There is a growing association of chronic RBD with neurodegenerative disorders – Parkinson’s disease, multiple system atrophy (MSA) or dementia-- as an early indicator of these conditions by as much as 10 years.
Patients with narcolepsy also are more likely to develop RBD.
Catathrenia, a rapid eye movement sleep parasomnia consisting of breath holding and expiratory groaning during sleep, is distinct from both somniloquy and obstructive sleep apnea. The sound is produced during exhalation as opposed to snoring which occurs during inhalation. It is usually not noticed by the person producing the sound but can be extremely disturbing to sleep partners, although once aware of it, sufferers tend to be woken up by their own groaning as well. Bed partners generally report hearing the person take a deep breath, hold it, then slowly exhale; often with a high-pitched squeak or groaning sound.