Salvador Dali would wake himself up the moment he fell asleep to capture and paint surreal dream images.

Many people argue that Salvador Dali is the father of surrealistic art. His art elaborates on juxtaposition, disposition, and morphing of objects. His most famous painting, Persistence of Memory, is instantly associated with him whenever it is seen. 

Surrealism is the use of dreamlike imagery, or, by definition, any imagery that is "surreal." Dali’s imagery and imagination were both so vivid and wild that people often wonder how he came to create the art he did. It turns out he had an oddly elaborate method.

Fascinated with the images he got as he was drifting off to sleep, Dali would place a tin plate on the floor and sit beside it in a chair, holding a spoon above the plate. He would relax and fall asleep, and the moment when he did, the spoon would fall and clash with the plate, waking him up with the dream images fresh in his mind. 

The logic behind this is that Dali was bringing the unconscious thoughts to the conscious level. The unconscious state is like a fire hydrant outside, while the conscious state is like a faucet in the house. 

Once that fire hydrant turns on, a constant supply of images flows in. Bringing the unconscious thoughts to that level is simply a matter of turning on that "fire hydrant," which, in Dali’s case, was accomplished by waking himself the moment he fell asleep. There is a blueprint to the process you can view at the source

(Source)