Rainbowfishes are certainly affected by external environmental factors which
in turn will trigger internal biological functions. One of the external
environmental factors that have been shown to play a significant role in
their life cycle is the photoperiod.
I don't really know if continuous light will have a detrimental effect on
them long term, but it does increase their metabolism and their spawning
behaviour. I guess if the light is not a bright light then it shouldn't
bother them too much as I would imagine that the natural nighttime is not
completely dark (moonlight etc).
And yes, they do sleep and if you sneak up on them in the dark and disturb
them they go into a mad panic. Also they have a different night-time
colouration as well.
Adrian R. Tappin
"Home of the Rainbowfish"