The disorder that we will discuss in the category of hypersomnias of central origin is narcolepsy. The hypersomnia disorders are those in which the primary complaint is daytime sleepiness (defined as the inability to stay alert and awake during the major waking episodes of the day, resulting in unattended lapses into sleep). In narcolepsy, excessive daytime sleepiness is debilitating and consists of repeated lapses into sleep episodes across the day. Patients typically sleep for a short duration and awaken refreshed, however, sleepiness occurs again within a few hours. Sleepiness is more likely to occur during activities that require little physical and cortical arousal, such as watching television. Sleep attacks are sudden and often irresistible due to their involuntary origins. We will see that the best way to conceptualize narcolepsy is by considering it an unwelcome flicking of a switch in the brain to turn on sleep (or rather, to turn off wake). In narcolepsy, sleep is not completely turned on however, only key portions, thus the switch is faulty.
Hypocretin, also known as orexin is the important mediator in the sleep-wake switch, but it is deficient in narcolepsy. The disorder is characterized by unique conditions apart from excessive daytime sleepiness, including cataplexy and vivid hallucinations.
Before we look at these two bizarre phenomenon we should note a quick word about sleep-onset in the form of a switch. In the main Circadian Rhythm and Sleep Architecture learning sections, we outline the critical structuring of sleep, and we discussed how that structure is embedded in a larger circadian rhythm cycle. We emphasize that sleep onset requires sleep preparatory hormones to be active on multiple fronts, and that the entire circadian rhythm ebb-and-flow structure that includes sleep within it does not run on 'master clock' oscillations alone, but has need to be synchronized with external cues such as light and darkness, as well as many of our consistent behaviors. All of this appears quite contrary to the notion of simple sleep and wake "switches" that can be augmented pharmacologically. When it comes to sleep onset, however, all of that preparatory work and critical internal functioning after sleep onset still requires a set of switch-like mechanisms.
Cataplexy is a medical condition in which strong emotion or laughter causes a person to suffer sudden physical collapse even though they remain conscious. It is a unique characteristic of narcolepsy and the sudden loss of muscle tone is often provoked by strong emotions that are often positive— such as laughter, pride, elation or surprise. Cataplexy may be localized to regions such as the lower limbs, neck, mouth or eyelids or it may include large skeletal muscle groups. Affected areas commonly include the knees, face, and neck. Respiratory muscles are not affected in cataplexy. Remember also that the respiratory muscles remain in tact during dream, so narcolepsy can be conceptualized as a form of REM-like episodes that intrude into the day through bad "switches". The duration of cataplexy is usually short, ranging form a few seconds to several minutes. Recovery is immediate.
Sleep paralysis and hypnagogic hallucinations (hallucinations occurring immediately prior to sleep onset) along with nocturnal sleep disruption commonly occur in patients with narcolepsy. The hypnagogic hallucinations are vivid perceptual experiences typically occurring right at sleep onset and can occur even for persons without narcolepsy if they are sufficiently sleep-deprived (as can accompanying sleep paralysis). These hallucinations are often accompanied by a realistic awareness of the presence of someone or something else in the room, and include visual, tactile, kinetic, and auditory phenomena. The accompanying feeling may be one of fear or dread that an intruder means harm but paralysis does not allow one to react. Because of this collection of specific and unique phenomenology, it has been often proposed that sleep paralysis and hypnagogic hallucinations are the basis for "alien abduction" intreprations from people who were either positive for narcolepsy and did not know it, or undergoing a period of severe sleep deprivation (which can invoke similar phenomenon).
For proper diagnosis, physicians ensure that the cause of the primary symptom must not be disturbed sleep from other sleep disorders or medical conditions, or a misaligned circadian rhythm, but due to "central" sleep-wake biological mechanisms. Other sleep disorders may be comorbid with a final diagnosis, but they must first be appropriately treated before the diagnosis is confirmed. The narcolepsy "tetrad" is below.