CNS hypersomnia is characterized by recurrent daytime sleepiness without the abrupt sleep attacks classically seen in narcolepsy. The daytime sleepiness is isolated. Historically, this syndrome has had a number of labels, including essential narcolepsy, independent narcolepsy, non-rapid eye movement sleep narcolepsy, functional hypersomnia, and harmonius hypersomnia. It is important to differentiate CNS hypersomnia from the narcolepsy syndrome, sleep apnea, or periodic leg movement syndrome.

Patients complain of long periods of daytime drowsiness that significantly impair their performance. Daytime drowsiness leads to prolonged naps, interrupted rarely by short awakenings and is frequently unrefreshing. Nocturnal sleep is often long and undisturbed. In the morning, awakenings are difficult, and sleep drunkenness is frequently noted. Patients are hard to awaken and can be aggressive and verbally and physically abusive during that twilight state if they are awakened, even at their own request. This may also occur when awakened from naps. Due to unrefreshing sleep and difficult awakenings, patients tend to fight sleepiness as long as they can.

CNS hypersomnia is a disabling disease understood even less than narcolepsy. Impairment is frequently severe. Treatment approaches include physician-directed medications and hygiene recommendations such as one nap daily (noon or late afternoon), no longer than 45 minutes; avoid alcohol, heavy meals, sleep deprivation; avoid shift work; sleep at least 8.5 hours per night.