Hypercapnea - An abnormally high arterial CO2 tension (PaCO2), usually defined as more than 48 mmHg.
Hypersomnia - A synonym for excessive daytime sleepiness. Although persons with hypersomnia chronically fall asleep when unstimulated, they do not necessarily require longer than average hours of sleep.
Hypnagogic Hallucinations - Vivid and dreamlike experiences, usually visual, which occur at sleep onset while the person is still aware.
Hypnic jerks (sleep starts) - Sudden, brief jerks that occur when a person is falling asleep, often associated with a subjective feeling of falling, a sensory flash, or a sleep-onset dream. Sleep starts are benign, but other, more complex motor behaviors may be associated with, or misinterpreted as, sleep starts that are symptomatic of a more serious condition (i.e., Restless Legs Syndrome).
Hypoglossus Nerve Stimulation (HGNS) - Treatment for obstructive sleep apnea that involves the use of an implanted nerve stimulator.
Hypothalamus - An area of the brain with a significant role in promoting wakefulness and sleep; it appears to be a key regulator (or “sleep switch”) of sleep and wakefulness. Wake-promoting hypocretin/orexin neurons are situated in the lateral hypothalamus.
Hypoxemia - Having lower than normal oxygen levels in the arterial blood.