in mammals and birds, sleep can be divided into two categories. in one of them, the eyes move rapidly. it is called rem-sleep (rapid eye movement). most dreams take place in this phase. as the body becomes relaxed and the eyes move while sleeping. this phase helps prepare one for the next day. rem-sleep occurs normally at intervals throughout the night, and the periods of rem-sleep increase in length in the second half of the night. it is often encountered 90 minutes after falling asleep, and continues to occur every 90 minutes. rem-sleep was first discovered in 195253.