Although sleep may seem like a steady state, it actually consists of several stages that cycle throughout the night. The types of brain waves (based on amplitudes and frequencies) determine the stages of sleep.  Lucid dreams occur in the 5th stage of sleep, known as the REM (Rapid Eye Movement) stage. As the name states, the most profound characteristic of REM sleep is the bursts of rapid eye movement while dreaming. REM sleep is marked by extensive physiological changes, such as accelerated respiration, increased brain activity, eye movement, and muscle relaxation. People dream during REM sleep, perhaps as a result of excited brain activity and the paralysis of major voluntary muscles.

One complete sleep cycle lasts about 90 to 100 minutes; therefore during an average sleep period a person will experience 4 to 5 complete sleep cycles. The sleep cycle begins with four stages of SWS (Slow-Wave Sleep) (also called NREM or Non-REM). These stages then quickly reverse, and are directly followed by the first REM period, roughly 90 minutes after falling asleep. Thus, the first REM period will last for about 10 minutes, as a new cycle begins about every 100 minutes. As the night proceeds, the length of stages 3 and 4 (also called delta or deep sleep) begins to wane, and the length of REM sleep increases, up to one full hour in length after a number of cycles. Therefore, as the night goes on, you dream for longer periods of time.  

Moreover, since the early 20th century, human sleep has been described as a succession of five recurring stages: four non-REM stages and the REM stage. A sixth stage, waking, is often included. Waking, in this context, is actually the phase during which a person falls asleep.

Sleep quality changes with transition from one sleep stage into another. Although the signals for transition between the five (or six) stages of sleep are mysterious, it is important to remember that these stages are, in fact, discretely independent of one another, each marked by subtle changes in bodily function and each part of a predictable cycle whose intervals are observable. Sleep stages are monitored and examined clinically with polysomnography, which provides data regarding electrical and muscular states during sleep.  

Brain Activity During Sleep  

The brain is not inactive during sleep. The electroencephalogram (EEG) tracings here show the patterns of electrical activity during different stages of sleep. Note that the brain waves of an alert person and those of a person in rapid eye movement (REM) sleep (when dreaming occurs) are similar in frequency and amplitude. In non-REM (NREM) sleep, the waves have a higher amplitude and a lower frequency, indicating that neurons in the brain are firing more slowly and in a synchronized fashion.

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The following table offers a brief summary of the stages of sleep:  

  Waking
The waking stage is referred to as relaxed wakefulness, because this is the stage in which the body prepares for sleep. All people fall asleep with tense muscles, their eyes moving erratically. Then, normally, as a person becomes sleepier, the body begins to slow down. Muscles begin to relax, and eye movement slows to a roll.