W. Kaiser, in the German Zoological Journal (77:297), reported on
experiments he conducted in 1984 on this subject.  He made extensive
observations of bees at night in observation hives illuminated with red
light (since bees don't see red light).
 
He found that bees in many areas of the hive at least rested.  They
formed clusters, stood motionless on empty cells, and remained in a
state of continuous muscle contraction.  Some bees even laid on their
sides.  The only bees which remained active all night were those on
brood combs (I guess they were on the night shift!)
 
Single worker bees observed in a special chamber containing empty comb
displayed similar behaviours.  The only signs of life were a series of
breathing movements in the abdomen, brief leg movements, and occasional
brief antenna movements.  At times, bees even crawled into an empty cell
and rested, lying on either their side or back!
 
Kaiser found two different resting states in bees, differentiated by how
quickly the bee reacted to infrared light stimulus (how quickly they
woke up?).
 
Kaiser didn't prove conclusively that bees slept (I'm not sure how he
would have been able to do that, really).  What he did do, however, was
show that bees displayed a series of behaviours that we normally
identify as related to sleep.
 
- Cliff Van Eaton, Tauranga, NEW ZEALAND