Some people hit the gym running in the morning while others groggily slam the snooze button. Neuroscientist Jeanne Duffy of Boston's Brigham and Women's Hospital believes much of the difference is rooted in biology—specifically, in the innate sleep-wake cycle known as the circadian rhythm.
Duffy and her colleagues studied sleep patterns among 17 men who lived in a controlled laboratory setting for one month. On average the period of the circadian rhythm is around 24 hours, but it varies from person to person. The researchers isolated the subjects from all cues that could indicate the time of day in order to bring out the natural circadian rhythm. Duffy then noticed an eye-opening pattern: The circadian periods of self-described "morning people" are consistently shorter than those of the "night people." These results bolster an earlier study, in which the same team found that evening types awake at or shortly after their peak hour of sleepiness, a stage in the circadian cycle marked by grogginess, low body temperature, and high levels of the hormone melatonin in the bloodstream. Morning types, in contrast, tend to wake well past the peak sleepiness phase, even though they get up earlier in the day.
"This explains why morning types are perkier and feel more alert when they first get up. Evening types are waking right when their alertness is at its absolute worst," Duffy says. These and other findings may elucidate why people's sleep patterns become generally more morninglike as they age, causing them to tire early. She also hopes her work will illuminate the mechanism behind delayed sleep phase syndrome, which causes mild insomnia that can lead to learning problems among many high-school students.