Lucid dreams usually happen during REM sleep. Sleep is not a uniform state, but is characterized by a series of stages (1, 2, 3, and 4, and REM) distinguished by certain physiological markers. REM sleep, stands for "Rapid Eye Movement" sleep, and is pronounced to rhyme with "them", not "R. E. M." Stages 1 through 4 are often lumped together under the label non-REM (NREM) sleep. Stages 3 and 4 are both referred to as "delta" sleep, for the large, low frequency brain waves evident in these stages.
Although this is certainly a gross oversimplification of the complexity of the physiological and mental events in sleep, research has demonstrated that most vivid dreaming occurs in REM sleep. It is characterized by an active brain, with low amplitude mixed frequency brain waves, suppression of skeletal muscle tone, bursts of rapid eye movements, and occasional tiny muscular twitches.
The sleep stages cycle throughout a night. The first REM period normally happens after a period of delta sleep, approximately 90 minutes after sleep onset, and lasts from about 5 to 20 minutes. REM periods occur roughly every 90 minutes throughout the night, with later REM periods occurring at shorter intervals and often being longer, sometimes up to an hour in length. Much more REM sleep occurs in the second half of the night than in the first.
How do we know that lucid dreaming happens in REM sleep? Most of the muscles of the body are paralyzed in REM sleep to prevent us from acting out our dreams. However, because the eyes are not paralyzed, if you deliberately move your "dream" eyes in a dream, your physical eyes move also. LaBerge's subjects slept in the laboratory, while the standard measures of sleep physiology (brainwaves, muscle tone and eye movements) were recorded. As soon as they became lucid in a dream, they moved their eyes in large sweeping motions left-right-left-right, as far as possible. This left an unmistakable marker on the physiological record of the eye movements. Analysis of the records showed that in every case, the eye movements marking the times when the subjects realized they were dreaming occurred in the middle of unambiguous REM sleep
