They're not exactly rival gangs, but most people who fall into one of these two categories feel pretty strongly about their preferred bedtimes. Although there are definitely exceptions to every rule, research has shown that early birds and night owls tend to exhibit specific personality traits and other tendencies. First, the good news: evening people are generally pegged as more outgoing, creative, intelligent and even having a better sense of humor than early risers. Unfortunately, their unique circadian rhythms have also been credited with higher incidence of ADHD, insomnia, depression, anxiety and addiction issues, to name a few.
By contrast, morning people are more optimistic, and therefore better able to fend off these problems, possibly due to their own genetic makeup. Larks also are more stable, conscientious and feel healthier and happier overall than night owls, perhaps because they experience more normal sleep patterns. Early birds are naturally synced up with the majority of business and educational schedules, helping them earn better grades and be generally more successful in the workplace, often resulting in higher wages [sources: Blaszczak-Boxe, Oakley, Randler]. Of course, there are exceptions to every rule, so it's totally possible to be a content night owl at the top of your corporate game.
Hormonal changes can affect our preferred sleep schedule throughout our lives, with adolescents generally moving into evening-preferred territory around age 13. Most people under 30 continue to enjoy a somewhat later schedule, but by age 50 or so the majority shift into early-riser status [source: Randler]. I'd like to refute that, but the older I get the less my stupid internal clock lets me laze about on weekend mornings. At this rate, I'll be popping out of bed at 5 a.m. by the time I retire.