I did a little more searching on google and found an article which explains how some of us are neither early birds (larks) or night owls, but instead, hummingbirds. Here's an excerpt:
What makes some of us "night owls"--people who perk up in the evening and don't go to bed until 2 a.m. (or even later)--while others are "larks"--early birds who wake up bright-eyed and ready to go at the crack of dawn? The answer lies mostly in our internal body clock, which is largely determined by our genes. In addition to driving our 24-hour (or circadian) sleep-wake cycle, this clock regulates hormone levels, body temperature, blood pressure, alertness and performance ability.
The cycles themselves are controlled mainly by a region within the brain's hypothalamus known as the suprachiasmatic nucleus. This area responds to signals from the retina, specifically, the light that travels from our eyes to our brain, which is the most important factor in orienting our bodies to daytime alertness and night-time sleep.
But that's not the whole story. The "owl" and "lark" categories account for just about 30 percent of the population, explains Michael Smolensky, Ph.D., a professor of environmental physiology at the University of Texas School of Public Health in Houston and co-author of The Body Clock Guide to Better Health (Henry Holt & Co., 2001). Smolensky says the remaining 70 percent of us are "hummingbirds"--people who can usually adapt when they need to, though it's easier or harder to do so depending on where you fall along the body-clock continuum.