A sleep disorder (somnipathy) is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning. A test commonly ordered for some sleep disorders is the polysomnography.
Apart from vision, human beings have many physiological and psychological responses to light. In rare individuals an atypical response may result in serious discomfort, disease, or injury. Some drugs have a photosensitizing effect. Properties of natural or artificial light that may abnormally affect people include:
Timing of light (upset of normal circadian rhythms, seasonal affective disorder, sleep disorders)
Intensity of light (photophobia, sunburn, skin cancer)
Wavelength of light ( in lupus, urticaria )
Rapid flickers in intensity of light may trigger or aggravate epilepsy or migraine headaches.
Other effects may include vertigo, or chronic fatigue syndrome.