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.

Electroencephalography (EEG) is the measurement of electrical activity produced by the brain as recorded from electrodes placed on the scalp.

Just as the activity in a computer can be understood on multiple levels, from the activity of individual transistors to the function of applications, so can the electrical activity of the brain be described on relatively small to relatively large scales. At one end are action potentials in a single axon or currents within a single dendrite of a single neuron, and at the other end is the activity measured by the EEG which aggregates the electric voltage fields from millions of neurons. So-called scalp EEG is collected from tens to hundreds of electrodes positioned on different locations at the surface of the head. EEG signals (in the range of milli-volts) are amplified and digitalized for later processing. The data measured by the scalp EEG are used for clinical and research purposes. In neurology, the main diagnostic application of EEG is for epilepsy but this technique is also used to investigate many other pathologies such as sleep-related disorders, sensory deficits, brain tumors, etc. In cognitive neuroscience, EEG is used to investigate the neural correlates of mental activity from low-level perceptual and motor processes to higher-order cognition (attention, memory, reading, etc).

In some cases, such as epileptic studies, when deeper brain activity needs to be recorded with more accuracy than provided by scalp EEG, clinicians use an invasive form of EEG known as intracranial EEG (icEEG) where electrodes are placed directly inside the skull. In some cases, a grid of electrodes is laid on the external surface of the brain, on dura mater yielding epidural EEG but in other cases, brain activity is recorded using deeper electrodes known as subdural EEG (sdEEG) and electrocorticography (ECoG). Because of the filtering characteristics of the skull and scalp, icEEG activity has a much higher spatial resolution than surface EEG. The technique is sometimes also referred to as stereotactic EEG (stereo-EEG or sEEG) to emphasize that it records from precise 3D locations defined by stereotaxy. However, since icEEG uses macro-electrodes for recording it can not detect single-neuron activity as it is feasible with neural implants based on micro-electrodes.

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.

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.