The Crick-Mitchison Theory (also known as reverse learning) is a biological theory of how and why dreaming occurs. Crick and Mitchison proposed that dreaming occurs during REM sleep when our brains are turned "off' from perceiving external stimuli. The brain then uses dreaming to go through information it acquired throughout the day and go through a process of reverse learning to forget useless information. So we are dreaming to forget irrelevant information. Our brain cannot handle the vast amount of information in perceives in a day. It would be pointless to store and retain irrelevant information so this theory proposes our brain cleans itself out by dreaming.