Ask Mr. Smarty Plants

Probably the reason you have not been able to find a definitive answer is that there isn't any. Although some studies have certainly been made, we couldn't find any scientific conclusions on the subject. We did, however, find the name of the function, from a website called Seeds Aside Word of the Week: Nyctinasty. Eschscholzia californica (California poppy) is just one of many plants on which this occurs. 

Since the purpose of the science fair project is no doubt to encourage students to learn how to do research and make conclusions on their own, we are going to suggest he begin by searching on the word "nyctinasty." We found the word by Googling on "flowers that close at night."  We would warn him, though, not to take everything on the Internet as absolute fact; a lot of it is opinion, not fact, a lot of it is speculation, and some of it is just wrong. He might want to list some of those speculations-like closing to hold in moisture (possible, but not proved) or the bloom was protecting its seeds from predators (maybe).