In mammals and birds, sleep can be divided into two categories. In one of them, the eyes move rapidly. This is called REM-sleep (from rapid eye movement). Most dreams take place in this phase. REM-sleep occurs normally at intervals throughout the night. The periods of REM-sleep increase in length in the second half of the night. REM-sleep was first discovered in 1952-53. The function of REM sleep is not well understood.