You shouldnt run your lights for more than 8-10 hrs max, if you do you will increase green alge growth. If you want light at night why not try moonlights? You can get them on Ebay for approx £10.00
Many breeders keep their fish under continuous lighted conditions, for what reasons I do not know. There is probably no direct harm that will come to your fish from continuous light (I know I keep mine on longer than most people, usually 14-15 hours a day), but realize that fish don't need light to survive, while other things growing in your tank may, so eliminating the source of light may also eliminate things like algae (on the glass and suspended) and the things which feed on algae. The choice is yours to make.