Sleep deprivation is a general lack of the necessary amount of sleep. This may occur as a result of sleep disorders, active choice or deliberate inducement such as in interrogation or for torture.

Insomnia is a symptom of a sleeping disorder characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. It is typically followed by functional impairment while awake. Insomniacs have been known to complain about being unable to close their eyes or "rest their mind" for more than...

Binaural beats or binaural tones are auditory processing artifacts, or apparent sounds, the perception of which arises in the brain independent of physical stimuli. This effect was discovered in 1839 by Heinrich Wilhelm Dove.

The brain produces a phenomenon resulting in low-frequency pulsations in the loudness of a perceived sound when two tones at slightly different frequencies are presented separately, one to each of a subject's ears, using stereo headphones. A beating tone will be perceived, as if the two tones mixed naturally, out of the brain. The frequency of the tones must be below about 1,000 to 1,500 hertz for the beating to be heard. The difference between the two frequencies must be small (below about 30 Hz) for the effect to occur; otherwise the two tones will be heard separately and no beat will be perceived.

Interest in binaural beats can be classified into two categories. First, they are of interest to neurophysiologists investigating the sense of hearing. Second, that binaural beats may influence the brain in more subtle ways through the entrainment of brainwaves and can be used to produce relaxation and other health benefits.

Sleep is a natural state of bodily rest observed throughout the animal kingdom. It is common to all mammals and birds, and is also seen in many reptiles, amphibians and fish. In humans, other mammals, and a substantial majority of other animals which have been studied ? such as fish, birds, ants, and fruit-flies ? regular sleep is essential for survival. However its purposes are only partly clear and are the subject of intense research.

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 polysomnogram.