370 Sleep-Wake Disorders
experiencing a sudden sleep “attack.” Unintent ional sleep episodes typically occur in low-
stimulation and low-activity situations (e.g ., while attending lectures, reading, watching
television, or driving long dist ances), but in more severe cases they can manifest in high-
attention situations such as at work, in meetings, or at social gatherings.
Associated Features Supporting Diagnosis
Nonrestorative sleep, automatic behavior, di fficulties awakening in the morning, and
sleep inertia, although common in hypersomnolence disorder, may also be seen in a variety
of conditions, including narcolepsy. Appr oximately 80% of indi viduals with hyper-
somnolence report that their sleep is nonres torative, and as many have difficulties awak-
ening in the morning. Sleep inertia, thou gh 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 hypersomnolenc e 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.
Prevalence
Approximately 5%–10% of individuals who cons ult in sleep disorders clinics with com-
plaints of daytime sleepiness are diagnosed as having hypersomnolence disorder. It is es-