Why Do We Have REM During Sleep?

Students often ask why our eyes move back and forth while we dream. "Why" is a tricky question. If you mean "why" in terms of what causes the eye movements to occur, the answer appears to be that during REM sleep there is a REM control center in the brainstem which gives off periodic bursts of activity that spread over wide areas of the brain. These chaotically arising signals activate other midbrain structures that control eye movements. If the REM center on the left side of the brain happens to send out a burst, the eyes will move to the left; if the right side center fires, the eyes will move to the right.

If you're asking "why" meaning what are the eye movements for, the answer is less clear. One theory is that it's one of the ways that the brainstem gives the forebrain the raw materials out of which to construct dreams; the forebrain recognizes that the eyes are moving and decides that something in their field of vision is moving, or that they're moving, and builds this information into the ongoing dream narrative. Interestingly, dreams turn out to be chock full of movement, much more than you would normally see in your waking life. So, maybe the rapid eye movements cue us that we are dreaming.

A second theory is called the "scanning hypothesis." The scanning hypothesis proposes that the eyes are actually following the action in a dream, much as your eyes follow the action on a movie screen. However, the anatomy of the brain doesn't seem to encourage this explanation; there are no major pathways that could carry clear information about the dream imagery back down to the brainstem so that it could send signals up to the midbrain to make the appropriate movements.