The state of sleep is measured objectively by a test known as polysomnography or PSG in humans. PSG consists of measurements of brain waves, eye movements and muscle tones. Brain waves are known as electroencephalography or EEG. EEGs are recorded from electrodes attached to the scalp. Eye movements are measured by electrodes attached on the face, two to three around eyes, also known as electrooculogram or EOG. Muscle tones are measured from two to three electrodes on the chin, known as electromyogram or EMG. By combining information from these electrodes, we can classify recordings into “sleep stages’.

Sleep can happen abruptly during the day when we are supposed to be fully awake and alert. This abrupt sleep is called microsleep or sleep attack. Microsleep may last from a couple of seconds up to about 30 seconds, shown here. Microsleep can occur without the person’s awareness of being asleep. Microsleep may occur with sleep deprivation and will usually entail “lapses” in performances. Lapses occur even when quicker responses are required, so they have been associated with life-threatening situations, like, sleeping while driving and possibly with other serious accidents. So sleep is very important to keep alert during the day!

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