- "Doc leaned over, holding a silver, penlight-sized device in front of Jennifer's face. The penlight-thing strobed a blue light in Jennifer's eyes. / Jennifer slumped in her seat, sound asleep. / 'Doc!' Marty objected. 'What are you doing?' / 'Relax, Marty,' Doc Brown reassured him. 'It's just a sleep-inducing alpha rhythm generator. She was asking too many questions. No one should know too much about their future.' Doc glanced at her again. Jennifer snored softly. 'This way, when she wakes up, she'll think it was all a dream.' "
- —From Back to the Future Part II by Craig Shaw Gardner (quote, page 16)
The sleep-inducing alpha rhythm generator was a device invented sometime before 2015 used to help induce sleep.
Emmett Brown used an alpha rhythm generator on Marty McFly's girlfriend Jennifer Parker to prevent her from remembering the events of time travel. He also used it on Marty Jr. However, because of its recent use on Jennifer, the device was not very effective and resulted in Marty Jr. arriving at the Cafe 80's 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 in Back to the Future Part II. 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".