By Kate Stinchfield

THURSDAY, April 23, 2009 ( — If you’re an early bird, you know the afternoon slump all too well. By the time that 4:30 p.m. meeting rolls around, you’re fading fast. You may blame it on a big lunch or a dark conference room, but a new study suggests that a drop in daytime alertness may actually be hardwired in your brain.

What’s more, night owls—people who stay up later and feel more awake in the evening—may have a naturally occurring advantage over early birds, according to a study published Thursday in the journal Science.

It seems that your alertness depends on the balance of two factors: your natural circadian rhythm and the intensity of pressure to sleep. Sleep pressure progressively increases throughout the day, meaning that the longer you've been awake, the less alert you should be. Brain scans suggest that night owls may be less sensitive than early birds to rising sleep pressure.

"If we have found what can appear as an advantage for evening types, it's that they are able to perform well after 10.5 hours spent awake, and that they're able to outperform morning types," says Philippe Peigneux, PhD, a professor of neuropsychology at the University Libre de Bruxelles in Belgium.

Next page: Caffeine to the rescue?

In the study, Peigneux and colleagues examined 16 extremely early risers, called larks, and 15 night owls. On average, the larks woke up and went to sleep about four hours before the night owls. The subjects followed their own preferred sleep-wake schedule, or chronotype, for a week prior to the study.

The volunteers reported to a sleep laboratory for two consecutive nights. The researchers checked their brain activity using functional magnetic resonance imaging (fMRI) as they performed visual attention tasks 1.5 hours after waking and 10.5 hours after waking.

The first scan showed no difference in brain activity performance, meaning that both larks and owls were equally alert 1.5 hours after getting up. The striking difference, though, came 10.5 hours after waking, when the night owls outperformed the morning chronotypes thanks to an activity increase in two brain regions—the suprachiasmatic area and the locus coeruleus.

"This shows that there are clear physiological differences between owls and larks and that they respond differently to accumulated sleep debt," says Mark Mahowald, MD, director of the Minnesota Regional Sleep Disorders Center. "Morning types are clearly affected by the duration of prior wakefulness, meaning they don't function as well starting in the late afternoon."

It's important to note that the volunteers were allowed to sleep and wake according to their own natural schedule, says Peigneux, so participants weren't overtired or skimping on sleep. And the larks' performance dropped toward the end of the afternoon, not late into the night.

"The science behind this study is really well done," says Ronald Kramer, MD, a Denver-based neurologist and a regional spokesperson for the American Academy of Sleep Medicine. "This shows that people's sleep needs and sleep timing is part of their biological makeup, much like weight. Your sleep preferences depend on biological predestiny, not necessarily lifestyle choice."

That isn't to say that your sleep schedule is set in stone. Like weight predisposition, says Dr. Kramer, you can make some lifestyle changes to help counteract your biological tendency. Most people fall somewhere between the two extremes anyway. "Being an early or late type is either end of a bell curve," says Dr. Mahowald. "The average person has a sleep schedule beginning at 11 p.m. or midnight and terminating at 6 or 7 a.m."

And if you happen to be a lark-like early riser, there are some simple ways to get a quick energy boost late in the afternoon. "Caffeine is a good short-term solution," says Dr. Mahowald. "And there's some evidence that bright light exposure can help keep you alert."

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