Kleine-Levin Syndrome or KLS, is a neurological disorder characterized by recurring periods of excessive amounts of sleep and altered behavior. At the onset of an episode the patient becomes drowsy and sleeps for most of the day and night (hypersomnolence), waking only to eat or go to the bathroom. When awake, the patients whole demeanor is changed, often appearing spacey or childlike. When awake, they experience confusion, disorientation, complete lack of energy, and lack of emotions. Individuals are not able to attend school or work or care for themselves. Most are bedridden, tired, and uncommunicative even when awake.

Most patients report that everything seems out of focus, and that they are hypersensitive to noise and light. In some cases, food cravings (compulsive hyperphagia) are exhibited. Instances of uninhibited hypersexuality during episodes (mostly males) and instances of depression (mostly females) have also been reported.