Hypersomnolence Disorder 371
months after the infection. Hypersomnolenc e can also appear within 6–18 months follow-
ing a head trauma. 
Genetic and physiological. Hypersomnolence may be fami lial, with an autosomal-
dominant mode of  inheritance. 
Diagnostic Markers
Nocturnal polysomnography demonstrates a no rmal to prolonged sleep duration, short
sleep latency, and normal to increased slee p continuity. The distribution of rapid eye
movement (REM) sleep is also normal. Sleep e fficiency is mostly greater than 90%. Some
individuals with hypersomnolence disorder have increased amounts of slow-wave sleep.
The multiple sleep latency te st documents sleep tendency, typically indicated by mean
sleep latency values of less than 8 minutes.  In hypersomnolence disorder, the mean sleep
latency is typically less than 10 minutes and fr equently 8 minutes or less. Sleep-onset REM
periods (SOREMPs; i.e., the occurrence of REM sleep within 20 minutes of sleep onset)
may be present but occur less than two time s in four to five nap opportunities. 
Functional Consequences of Hypersomnolence Disorder
The low level of alertness that occurs while an individual fights the need for sleep can lead
to reduced efficiency, diminished concentration, and poor memory during daytime activ-
ities. Hypersomnolence can lead to significan t distress and dysfunction in work and social
relationships. Prolonged nocturnal sleep and difficulty awakening can result in difficulty
in meeting morning obligations, such as arrivi ng at work on time. Unintentional daytime
sleep episodes can be embarrassing and even dangerous, if, for instance, the individual is
driving or operating machinery when the episode occurs. 
Differential Diagnosis