I read somewhere that Mick Jagger (or maybe it was Keith Richard) wrote the riff of Satisfaction in his sleep.

He woke up, grabbed a tape recorder and went "Bum bum. Bumbumbuummm bumbumbum." And sang the one line "I, can't, get noo, sat-tis-fac-tion." And then went back to sleep.

Forgot about it the next morning till they were at band practice and someone found the tape and played it.