There's a blog post on the New York Times that talks about a study on bedtime and behavior, and children with a regular bedtime are better behaved. I don't think this is any breakthrough finding, but it did make me stop and think about my kids' bedtimes. Their bedtimes are not a hard rule, but we generally don't allow them to stay up past 9:00 on school nights. Sometimes, they go to bed earlier on their own, other times they need a reminder.
Here's the article: http://well.blogs.nytimes.com/2013/10/14/regular-bedtimes-tied-to-better-behavior 
Do your kids have a bedtime? How strict are you with it?