driving or operating machinery when the episode occurs. 
Differential Diagnosis
Normative variation in sleep. “Normal” sleep duration varies considerably in the general
population. “Long sleepers” (i.e., individuals who require a greater than average amount
of sleep) do not have excessive sleepiness, sleep  inertia, or automatic behavior when they
obtain their required amount of nocturnal sleep . Sleep is reported to be refreshing. If social
or occupational demands lead to shorter nocturnal sleep, daytime symptoms may appear.
In hypersomnolence disorder, by contrast, symp toms of excessive sleepiness occur regard-
less of nocturnal sleep duration. An inad equate amount of no cturnal sleep, or behaviorally
induced insufficient sleep syndrome,  can produce symptoms of daytime sleepiness very similar
to those of hypersomnolence. An average slee p duration of fewer than 7 hours per night
strongly suggests inadequate nocturnal sleep , and an average of more than 9–10 hours of
sleep per 24-hour period suggests hypersomnole nce. Individuals with inadequate noctur-
nal sleep typically “catch up” with longer sleep durations on days when they are free from
social or occupational demands or on vaca tions. Unlike hypersom nolence, insufficient
nocturnal sleep is unlikely to persist unabat ed for decades. A diagnosis of hypersomno-
lence disorder should not be made if there is  a question regarding the adequacy of noctur-
nal sleep duration. A diagnostic and therapeutic trial of sleep extension for 10–14 days can
often clarify the diagnosis.
Poor sleep quality and fatigue. Hypersomnolence disorder should be distinguished
from excessive sleepiness related to insufficient  sleep quantity or quality and fatigue (i.e.,
tiredness not necessarily relieved by increase d sleep and unrelated to sleep quantity or