My common pleco used to love cucumber, zucchini, and yellow summer squash. Cut a nice big hunk and either get a veggie clip that suctions to the side, or use a rubber band and attach it to a piece of slate or heavy sinking ornament. Put it in at night after lights off and your pleco will munch away.
Fish don't really sleep, but pleco's are more nocturnal so they feed at night when lights are out. The other fish become more inactive and stay kind of still when it gets dark, this gives your pleco the opportunity to eat.