Dreams occur during sleep, which consists of four stages. Three non-REM stages in which brain waves and muscle activity are slowing down, ultimately reaching slow-wave sleep. The fourth, REM, stage is characterized by the rapid eye movement (thus the name REM) and the occurrence of alpha waves. This is the stage in which we usually dream.

A dream is a series of images, ideas, emotions, and sensations occurring involuntarily in the mind during certain stages of sleep. They mostly occur during REM sleep, when the brain activity is high, in some cases as high as when we are awake. Dreams also occur in non-REM sleep, but are limited to isolated images, sounds or stimuli.

Length of dreams varies depending on whether they occurred during non-REM or REM sleep, and can last from a few seconds to half an hour. If we wake up during REM sleep, we are most likely to remember our dreams. The average person has about 3 to 5 dreams per night. The longer we sleep dreams tend to last longer. During a typical lifetime, we spend about six years dreaming. The earliest recorded dreams were acquired from materials dating back approximately 5000 years, in Mesopotamia, where they were documented on clay tablets. The recording and interpretation of dreams has always been part of human culture as well as assigning different roles and meanings to them. Except in humans, REM sleep and the ability to dream have been also recorded in animals. All mammals are believed to experience REM sleep. The duration of REM is the shortest in dolphins, and the longest in armadillo. Dolphins have the shortest REM sleep cycle, while armadillos have the longest.