The sleep-inducing alpha rhythm generator was a device invented sometime before 2015 used to help induce sleep. Doc used this on Marty's girlfriend Jennifer Parker to prevent her from remembering the events of time travel. He also used it on Marty McFly Jr. However, because of its recent use on Jennifer, it was not very effective and resulted in Marty Jr. arriving at the Cafe 80s where Marty was pretending to be his own son to confront Griff Tannen.
Behind the scenes
- The device that Doc demonstrated apparently had two lights that would shine into a person's eyes and send a signal similar to the brain wave pattern shown during the early stages of sleep.
- As others have pointed out , this is not one of Doc's inventions, but an "EZ Sleep" product that he purchased.
- Patents have been granted for alpha-rhythm generators, though not for anything resembling the device used by Doc. Most recently, patent #7,155,285 was issued in 2006 by the United States Patent Office  for an "apparatus for inducing energies of alpha rhythm to the human body".