Before the twentieth century, sleep was thought of as just a period of restful inactivity because there was no clear way to study brain activity. But since then, the invention of the electroencephalograph by German psychiatrist Hans Berger in the 1920s gave sleep researchers a tool for studying brain activity. These brain activities that they study are called brain waves. Today's scientists research a number of physical functions during sleep such as eye movement, muscle movement, breathing rate, air flow, pulse, blood pressure, amount of exhaled carbon dioxide, body temperature, and breathing sounds. There are two basic types of sleep: REM sleep, also known as rapid eye movement sleep or active sleep, and NREM sleep, or non-rapid eye movement sleep, also known as quiet sleep.
             While you are preparing for bed you are up and reasonably alert so your brain produces small, fast brain waves called beta brain waves. When your head hits the pillow, your muscles begin to relax and your brain's electrical activity starts to slow down making the brain produce slower and larger brain waves called alpha brain waves. During this stage, you may start to experience odd but realistic sensations such as your name being called, or you may feel a falling sensation or see kaleidoscope-like patterns. These experiences are called hypnagogic hallucinations. The most common of these hallucinations is the sensation of falling. This is often followed by a myoclonic jerk which is an involuntary muscle spasm of the whole body that jerks the person awake. You are most likely to experience this when you are very tired or under stress.
             First 90 Minutes of Sleep and Beyond
             Over the course of the night, your sleep patterns are fairly consistent. As you go into NREM sleep, you go through four different stages. Each stage you go through is characterized by a decrease in brain and body activity.
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