A dream is a collection of images and ideas that occur involuntarily during certain periods of repose. When you first drift off, your heart rate slows, your temperature drops, and your brain is busy processing the day’s events. During this initial sleep stage, dreams are made up of flashes of thoughts and images from your waking life: what you ate for lunch, a phone call you made during the day, the movie you watched before bed. You rarely remember these dreams unless you wake up during them.
After about 90 minutes, you fall into the rapid eye movement (REM) stage of sleep, where vivid, often surreal dreams occur. The amygdala, the area of the brain responsible for processing emotions, and the hippocampus, the seat of memory, are both active, which is why REM dreams have a storylike quality and are the ones you tend to remember the next day and recount to friends. If you get six to eight hours of sleep, you experience four to five REM periods of various lengths, all of which are dream filled (though you probably won’t remember most of them).