Exaggerated sleepiness and desire to go to sleep. Often the hypersomnist is unable to stay awake during the day in inactive understimulating situations.
Most often hypersomnia is an effect of the different varieties of insomnia, for example sleep apnea, where the sleeper has difficulty breathing during the night, because of the upper respiratory tract being too narrow. In this case it closes when the person relaxes during the initial stages of sleep. The oxygen deprivation caused by this has the effect of waking the person up as soon as she/he starts falling asleep, only to fall asleep again as soon as normal breathing sets in. Usually this means that the person is not aware of waking up and has no recollection of it and the behaviour repeats time after time during the whole night. This means that the sleep has virtually no effect at all and no recreative value.
Other reasons might also very well be the cause of apnea, for instance darkness, monotony and jet lag.
See narcolepsy.