Sleep is a state of consciousness that is essential to your daily mental and physical functions. While no one has been able to determine exactly why people sleep, studies have shown that sleep allows your body to restore itself after a time of wakefulness. During sleep the body has time to repair itself (repair cells, strengthen the immune system, digest, and store the mental processes required in learning) after the damaging effects of everyday activities.

Until the early 1950s when the electroencephalograph (EEG) was invented, sleep was thought to be a time-out period during which the brain shuts down. However, the EEG made it possible to study the brain waves of people sleeping and found that the brain was anything but shut down during sleep. Researchers discovered that there are two basic types of sleep: REM (rapid eye movement) sleep, which is associated with higher levels of body and brain activity, and non-REM sleep, during which body and brain activity slows down. By recording the electrical activity of the brain, researchers were able to determine that the brain goes through four stages of activity during sleep:

  • Stage 1 — During the first stage, you begin to relax and your body prepares for sleep. According to the EEG, your brain first emits alpha waves, which show a slow, regular rhythm. As you drift further into stage 1, the waves become more irregular and slower theta waves begin to emerge. You are in a state of light sleep from which you can easily be awakened.

  • Stage 2 — During the second stage, your brain emits irregular, short bursts of waves that contain peaks, called sleep spindles. Slow theta waves are predominant during this stage, but even slower brain waves known as delta waves begin to emerge as well. Even though you are asleep at this stage, you may not think you are. Background noise isn't likely to awaken you.

  • Stage 3 — This third stage takes you progressively deeper into true sleep. Delta waves represent approximately 20 percent of the activity during this stage. Your body has relaxed to the point that your pulse rate and breathing slow down. You are more difficult to arouse during this stage.

  • Stage 4 — During the fourth stage, the alpha waves of stage 2 have all but left the scene and the delta waves have taken control. When the level of delta waves reach 50 percent, you are said to have entered stage 4 sleep. You are in a very deep sleep at this point and largely oblivious to outside noises. It will require an outside stimulus such as a loud alarm clock to awaken you.

  • Once you reach the final stage, you will travel backward through the stages until you reach the first. However, instead of entering the period of semiwakefulness, your body enters REM sleep. REM sleep gets its name from the rapid eye movement that occurs during this stage. The EEG shows that during REM sleep, your brain is just as active as it is when you are wide-awake during the day. During REM, your blood pressure rises, your heart-beat increases, you breathe faster, and you will likely dream.

    You will alternate from REM sleep to stages 2, 3, and 4 several times throughout the night. As the hours pass, the time spent in the stages often gets shorter and shorter and time spent in REM gets longer and longer.

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