Circadian Rhythm Disorders
Circadian rhythm disorders are conditions that may result when an individual’s internal biological clock is out of sync with external time cues, including the natural dark-light cycle. This may occur in total blindness, with shift work or jet lag, or due to advanced or delayed sleep phase syndrome. The mismatch may lead to insomnia or hypersomnia at inappropriate times.
Narcolepsy
Narcolepsy is a sleep disorder characterized by four classic symptoms: excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. The sleepiness can be profound and may lead to falling asleep in inappropriate situations. Cataplexy is a loss of muscle tone in response to an emotional stimulus, such as surprise or laughter. Sleep paralysis is an inability to move one's body while being awake, usually while falling asleep or awakening. Hypnagogic hallucinations are vivid dream-like auditory, visual, or tactile sensations that occur while falling asleep.
Chronic Fatigue Syndrome
Chronic fatigue syndrome (CFS) is characterized by unexplained prolonged fatigue that is not improved by rest and may be worsened by physical or mental activity. This fatigue can be severe and incapacitating, causing a substantial reduction in daily activities. It may require adaptations to conserve energy in extreme fatigue. There are a number of associated symptoms, and other medical conditions must be excluded before CFS can be diagnosed.
Jet Lag
Jet lag is a temporary condition caused by rapid travel across time zones -- as may occur with jet trips -- and may leave an individual experiencing fatigue, insomnia, nausea, or other symptoms as a result of the internal circadian rhythm, or body clock, being misaligned with local time.
Seasonal Affective Disorder
Seasonal affective disorder is a recurrent mood disorder associated with depression and excessive sleepiness during winter months. It is caused by a lack of bright light reaching the biological clock in the suprachiasmatic nucleus, a small region of the brain. Its treatment is the use of a light box to artificially extend the day length.
