Dreams are successions of images , ideas , emotions , and sensations that occur involuntarily in the mind during certain stages of sleep . [ 1 ] The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation, as well as a subject of philosophical and religious interest, throughout recorded history. The scientific study of dreams is called oneirology . Scientists believe that, in addition to humans, birds and other mammals also dream. [ 2 ] Dreams mainly occur in the rapid-eye movement (REM) stage of sleep —when brain activity is high and resembles that of being awake. REM sleep is revealed by continuous movements of the eyes during sleep.
Hot tags Spacecraft Climate Change Animals Health Astronomy (Space) Editor's choice Polar ice Arctic to green as Antarctic sea ice grows Monday, 1 April 2013 In the coming decades, global warming will cause grass, shrubs and trees to thrive in Arctic soil stripped of ice and permafrost, while sea ice around Antarctica will grow, according to new research published today. Photo gallery