Their sleep cycle can be different than just their circadian rhythm: if you sleep deprive ’em, they need more sleep. A bunch of molecular similarities, too (i.e. some sleep-related drugs do the same things to flies).
Paul J. Shaw, Chiara Cirelli, Ralph J. Greenspan, and Giulio Tononi. .
Science 2000; 287 (5459) : 1834-1837.
Newer review article: Joan C. Hendricks. Genetic Models in Applied Physiology: Invited Review: Sleeping flies don’t lie: the use of Drosophila melanogaster to study sleep and circadian rhythms. J Appl Physiol 94: 1660-1672, 2003. 10.1152/japplphysiol.00904.2002