People typically dream 4-5 times per night, with over 95% of dreams being forgotten. Dreams mainly occur during REM sleep, which accounts for around 90% of dreams. REM sleep increases throughout the night as sleep time increases. There are several theories about the meaning of dreams, including Freud's psychoanalytic theory that dreams reveal unconscious desires, the problem-solving theory that dreams have no inherent meaning but reflect how problems are processed, and Hobson and McCarley's activation-synthesis theory that dreams have no meaning and are simply a product of random brain activation during sleep. Researchers study dreams using sleep laboratories that can monitor physiological activity and brain waves during sleep.