Stages of sleep were discovered in the 1950s and 60s—when researchers first placed electrodes on the heads of sleepers and recorded electrical activity all night long. Scientists quickly discovered that there were different types of sleep. Deep, or slow wave sleep, preceded periods of lighter sleep. Also, a period of dreaming (REM) always came at the end of a “sleep cycle.” Sleep cycles in humans last an hour and a half, which is why we tend to sleep in multiples of 90 minutes. Count the number of hours you sleep tonight and you will see that this is true.

Today it is generally accepted that deep sleep repairs the body, while REM repairs the mind. Interestingly, these same sleep patterns—deep sleep, light sleep, and REM—have been found in every warm-blooded creature on earth. In addition to repairing the brain, REM is strongly implicated in growing it. All neurologically complex, warm-blooded creatures have more REM when they are young than when they are mature.

Despite recent advances in understanding sleep, we still know very little about it—especially REM. The primary reason sleep is elusive to us is because psychologically, by definition, it is an unconscious state. Things still happen to us during sleep—light sleep, deep sleep, and dreaming—but without consciousness, there’s no “I” there to experience it in the present tense. This is why we always have to “look back” on our experience of sleep. “We” weren’t really there to experience it the first time around.

When we are unconscious, our memory suffers as well. This explains why we typically don’t remember much—aside from the occasional dream—from our usual seven or eight hours of sleep. When we are unconscious we also lose our sense of time. Why? Because there’s no one “there” to notice its passing.

An exception to this absence of consciousness occurs during lucid dreams. A lucid dream is defined as a dream in which the dreamer is aware he or she is dreaming—while the dream is occurring. Lucid dreams are rare, but they give us fabulous glimpses behind the veil which normally shrouds the mystery of sleep. For more information on lucid dreams, visit the Lucid Dreams section on Ask the Dream Doctor .