Night Owls Stay Alert Longer Than Early Birds

http://news.nationalgeographic.com/news/2009/04/090423-night-owls-brains.html


Night Owls Stay Alert Longer Than Early Birds
Anne Minardfor National Geographic News
April 23, 2009
Score one for the night owls—sort of.
Early birds may be chipper in the morning, but they mentally wear out faster, a new brain-scan study reveals.
Scientists monitored the brain activity of self-described early birds and night owls in a sleep lab.
(Related: "Early Risers Have Mutated Gene, Study Says.")
The team, led by Christina Schmidt of the University of Liège in Belgium, also took hourly saliva samples to measure the sleepers' levels of melatonin, a hormone thought to help naturally regulate sleep cycles in mammals.
Both night owls and early birds were allowed to stay on their preferred sleep schedules, but each group was awake for the same number of hours each day.
An hour and a half after waking, the groups scored the same on tests that required them to pay attention to a task.
But ten hours after waking, early birds showed reduced activity in brain areas linked to attention compared with the night owls. The "morning people" also felt sleepier and performed more slowly on tests.
Furthermore, as the day wore on, early birds showed less activity in a region deep in the brain involved in the so-called circadian master clock, which regulates our daily cycles of alertness.
The finding suggests that people tend to favor mornings or nights based at least in part on how they react to a kind of competition in the brain.
Circadian hormones, which keep us alert while awake, can get overridden by sleep pressure, a physiological pull that causes us to get sleepier the longer we're awake.
But while night owls seem to handle sleep pressure better, the late-to-bed strategy might backfire outside the lab, noted study co-author Philippe Peigneux, also of the University of Liège.
"Morning types may be at an advantage, because their schedule is fitting better with the usual work schedule of the society," he said.
"It may represent a problem for evening types obliged to wake up early while having difficulties going to bed in the evening, eventually leading to a sleep debt."
Findings published in this week's issue of the journal Science.

Evolution psychology: why would it have been beneficial for night owls to stay more alert and longer? why would this same alertness not benefit early birds?

My guess is that as the day goes on it gets darker, and therefore those up during those times of decreased light would need to be more alert to danger.