Plants as we know them cannot be defined to sleep, but an intelligent plant would probably have a similar type of dormancy. It has recently been shown that jellyfish sleep, and they do not have brains. It is possible that alien life might not require a sleep state to keep their nervous system (equivalent) running smoothly, but I see no reason why that would be less likely for one that photosynthesized (technically, no alien can be a plant or animal, kingdoms that categorize the products of terrestrial evolution; non-technical usage will, of course, refer to alien plant-analogs as "plants" and alien animal-analogs as "animals") than for one that does not. Earthly plants follow rhythms of activity (and hence inactivity).