Dreams are experiences during sleep that are internally generated by the
brain. Dreaming is a common phenomenon during normal sleep and dreaming may
occur in all sleep stages. Brain is active during sleep, but the perceptual
connection to environment is mostly turned off. 
During dreaming, the contents of dreams can, however, be affected by
both external and internal stimuli. Dream recall is associated with higher
frequency brain activity during sleep, typical of REM-sleep and arousals.

During parasomnias, dream experiences may be disturbed, behaviorally
manifested or mixed with waking reality. Parasomnias are sleep disorders
characterized by incomplete transitions between sleep and wake. Abnormal motor,
sensory, or behavioral manifestations of parasomnias occur at sleep onset,
within sleep, or during arousal from sleep. Various sensory stimuli are able to
disturb sleep causing arousals or partial awakenings typical of parasomnias.

Differential diagnosis of parasomnias include patient report of recalled
and enacted dream content, timing and onset-age of parasomnia episodes,
witnessed sleep behavior, and polysomnographic findings about the sleep stage
and the mechanism of episodes. In adults, parasomnias may have significant
negative effects on wellbeing and even violent consequences. A patient case
with severe parasomnia overlap disorder and its treatment is presented.