The seven lamps of sleep. In the mansion of the Knight of the Black Castle were seven lamps, which could be quenched only with water from an enchanted fountain. So long as these lamps kept burning, everyone within the room fell into a deep sleep, from which nothing could rouse them till the lamps were extinguished. (See Rosana.) (The Seven Champions of Christendom, ii. 8.)
The Romans are said to have preserved lamps in some of their sepulchres for centuries In the papacy of Paul III. one of these lamps was found in the tomb of Tullia (Cicero's daughter), which had been shut up for 1,550 years. At the dissolution of the monasteries a lamp was found which is said to have been burning 1,200 years. Two are preserved in Leyden museum.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894