I have a lot of tanks in my room and I "take a hint" when one of them tries to sleep in his little sea urchin thingy. He has one of those clip-on lights that has a white light (day) and a softer blue light. When it's getting later, I turn it to blue but aim the light at the back of the tank so that he has darkness in his favorite sleeping places. Then when I go to sleep I turn all their lights off.
Now, I did see little lights with a suction cup that can go in the water, at Petsmart around $14-$16. It comes in white light or green, or blue .. . I would think if you really want a light on you maybe could look for something like that in blue, but be sure to aim it so the fish does have a dark corner to go to if it wants.
Just from observing them, I feel that my bettas prefer darkness when it's time for them to sleep.
(This is the type of clip-on light I mean: http://www.amazon.com/gp/product/B00...ls_o06_s00_i00
I'm not promoting these exact ones, there are probably better kinds out there and I doubt either of these can grow plants but these are just examples)
This is the little suction light I mentioned: