Electroencephalogram (EEG) traces of brain wave activity reveal that, in general terms, non-REM sleep is characterized by very slow but relatively high amplitude or high voltage oscillations (with the frequency gradually slowing and the amplitude increasing as sleep deepens), while REM sleep shows a much faster and lower amplitude trace, much more similar to normal waking activity (see diagram above). Also, brain waves during non-REM sleep tend to be highly synchronized , and those during REM sleep much more unsynchronized . Electrooculogram (EOG) traces of eye movement indicate rapid eye movements during REM sleep , and little or no eye movement during non-REM sleep . Electromyogram (EMG) traces of skeletal muscle activity show that, while the body is effectively completely paralyzed during REM sleep , the body does make some limited movements during non-REM sleep , including a major change in body position about once every twenty or thirty minutes on average. Based on these characteristics, early sleep researcher William C. Dement has described non-REM sleep as an idling brain in a moving body, and REM sleep as an active hallucinating brain in a paralyzed body.
Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the circadian clock. People naturally feel most sleepy at two times of the day about 12 hours apart—for example, at 2:00 . and 2:00 . At those two times, the body clock "kicks in." At about 2 . (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 . (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.