| First Stage | The first stage is a period between wakefulness and sleep, which lasts between 5-15 mins. In this stage the brain produces high amplitude theta waves, which are very slow brain waves. |
| Second Stage | The second stage lasts for approximately 20 minutes. The brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. Body temperature starts to decrease and heart rate begins to slow. | Third Stage | In the third stage deep, slow brain waves known as delta waves begin to emerge. This stage is a transitional period between light sleep and a very deep sleep. |
| Fourth Stage | The Fourth stage lasts 30 minutes and is sometimes referred to as delta sleep because of the slow brain waves known as delta waves that occur during this time. Bed-wetting and sleepwalking may occur in this stage. | Fifth Stage | Most dreaming occurs during the fifth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterized by eye movement, increased respiration rate and increased brain activity. REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due because of increased brain activity, but voluntary muscles become paralyzed. |
During REM sleep, several physiological changes also take place. The heart rate and breathing quickens, the blood pressure rises, we can't regulate our body temperature as well and our brain activity increases to the same level (alpha) as when we are awake, or even higher. The rest of the body, however, is essentially paralyzed until we leave REM sleep. This paralysis is caused by the release of glycine, an amino acid, from the brain stem onto the motoneurons (neurons that conduct impulses outward from the brain or spinal cord). Because REM sleep is the sleep stage at which most dreaming takes place, this paralysis could be nature's way of making sure we don't act out our dreams.