Moon-struck persons. The Romans believed that the mind was affected by the moon, and that “lunatics” were more and more frenzied as the moon increased to its full. (See Avertin.)
“The various mental derangements ... which have been attributed to the influence of the moon, have given to this day the name lunatics to persons suffering from serious mental disorders.” —Crazier. Popular Errors. chap iv. p. 53.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894