- Science has proven to an extent that all mammals dream.
- Other hypotheses about the function of dreams, including:
- Dreams allow the repressed parts of the mind to be satisfied through fantasy while keeping the conscious mind from thoughts that would suddenly cause one to awaken from shock.
- Sigmund Freud suggested that bad dreams let the brain learn to gain control over emotions resulting from distressing experiences.
- Carl Jung suggested that dreams may compensate for one-sided attitudes held in waking consciousness.
- Sandor Ferenczi proposed that the dream, when told, may communicate something that is not being said outright.
- Dreams regulate mood.
- Hartmann says dreams may function like psychotherapy, by "making connections in a safe place" and allowing the dreamer to integrate thoughts that may be dissociated during waking life.
- More recent research by psychologist Joe Griffin, following a twelve-year review of data from all major sleep laboratories, led to the formulation of the expectation fulfilment theory of dreaming, which suggests that dreaming metaphorically completes patterns of emotional expectation in the autonomic nervous system and lowers stress levels in mammals.