Hypersomnolence is a broad diagnostic term and includes symptoms of excessive quantity of sleep (e.g., extended nocturnal sleep or involuntary daytime sleep), deteriorated quality of wakefulness (i.e., sleep propensity during wakefulness as shown by difficulty awaken- ing or inability to remain awake when required), and sleep inertia (i.e., a period of im- paired performance and reduced vigilance following awakening from the regular sleep episode or from a nap) (Criterion A). Individuals with this disorder fall asleep quickly and have a good sleep efficiency (>90%). They may have difficulty waking up in the morning, sometimes appearing confused, combative, or ataxic. This prolonged impairment of alert- ness at the sleep—wake transition is often referred to as sleep inertia (i.e., sleep drunkenness).
It can also occur upon awakening from a daytime nap. During that period, the individual appears awake, but there is a decline in motor dexterity, behavior may be very inappro- priate, and memory deficits, disorientation in time and space, and feelings of grogginess may occur. This period may last some minutes to hours.
The persistent need for sleep can lead to automatic behavior (usually of a very routine, low-complexity type) that the individual carries out with little or no subsequent recall. For example, individuals may find themselves having driven several miles from where they thought they were, unaware of the ”automatic” driving they did in the preceding minutes.
For some individuals with hypersomnolence disorder, the major sleep episode (for most individuals, nocturnal sleep) has a duration of 9 hours or more. However, the sleep is often nonrestorative and is followed by difficulty awakening in the morning. For other individ- uals with hypersomnolence disorder, the major sleep episode is of normal nocturnal sleep duration (6—9 hours). In these cases, the excessive sleepiness is characterized by several un- intentional daytime naps. These daytime naps tend to be relatively long (often lasting 1 hour or more), are experienced as nonrestorative (i.e., unrefreshing), and do not lead to improved alertness. Individuals with hypersomnolence have daytime naps nearly everyday regard- less of the nocturnal sleep duration. Subjective sleep quality may or may not be reported as good. Individuals typically feel sleepiness developing over a period of time, rather than experiencing a sudden sleep ”attack.” Unintentional sleep episodes typically occur in low- stimulation and low-activity situations (e.g., while attending lectures, reading, watching television, or driving long distances), but in more severe cases they can manifest in high- attention situations such as at work, in meetings, or at social gatherings.
Nonrestorative sleep, automatic behavior, difficulties awakening in the morning, and sleep inertia, although common in hypersomnolence disorder, may also be seen in a variety of conditions, including narcolepsy. Approximately 80% of individuals with hyper- somnolence report that their sleep is nonrestorative, and as many have difficulties awak- ening in the morning. Sleep inertia, though less common (i.e., observed in 36%—50% of individuals with hypersomnolence disorder), is highly specific to hypersomnolence. Short naps (i.e., duration of less than 30 minutes) are often unrefreshing. Individuals with hy- persomnolence often appear sleepy and may even fall asleep in the clinician’s waiting area.
A subset of individuals with hypersomnolence disorder have a family history of hy- persomnolence and also have symptoms of autonomic nervous system dysfunction, in— cluding recurrent vascular-type headaches, reactivity of the peripheral vascular system (Raynaud’s phenomenon), and fainting.
Approximately 5%—10% of individuals who consult in sleep disorders clinics with com- plaints of daytime sleepiness are diagnosed as having hypersomnolence disorder. It is es- timated that about 1% of the European and US. general population has episodes of sleep inertia. Hypersomnolence occurs with relatively equal frequency in males and females.