When we fall asleep, our brains invent incredibly realistic and memorable dreams. Sometimes those dreams are dark and terrifying, making falling asleep something to fear. On this week's Please Explain, we'll find out what goes on inside our brains as we sleep. Dr. Andrew Gerber, a Neuroscientist and Clinician at Columbia University, and David K. Randall, journalist and author of Dreamland: Adventures in the Strange Science of Sleep , explain the neuroscience of dreams and nightmares.

Why do we dream? Do dreams have a purpose? When the brain is asleep it's inundated by impulses and senses and experiences, and the brain puts them together. A necessary function of sleep, especially of REM sleep, is to consolidate the information that was gathered in the previous day and even in the more distant past, into coherent, long-term memories that get stored for later use. That's what dreams are. Dr. Gerber explained that the general scientific theory is that “Every night when we sleep our brains are consolidating what we should remember and letting go what we don’t.”

According to  Dr. William Domhoff, a professor at UC Santa Cruz who collected 50,000 dream logs, most dreams are by and large negative. “People are attacked, people don’t like you, it’s kind of like the worst days of middle school,” said David Randall.

Are dreams and nightmares different? Dreams that have the same basic structure—a narrative, imagery, a plot—have the same kind of neural substrate to them, whether they be positive or negative. When you wake up with a generally bad feeling but not with any recollection of a bad dream, it might mean that you had a dream with a different kind of structure that involved different parts of the brain.