Meredith said that the circadian system developed in all living beings as a survival tool, and in response to the rotation of the earth and the alternating cycles of darkness and light. Early humans needed to be awake when the animals they hunted were foraging in the open, and to sleep when their prey retreated to their caves, trees or nests for the night.
She likens these rhythms to "the guy with the megaphone telling you when to sleep and when to wake up.
"The sleep system responds to those cues," she said. "The circadian system is so important and so powerful that the sleep system can't override it, unless the person is significantly sleep-deprived."
And when these cycles become disrupted by such external factors as jet lag or the spring/fall time change, there are consequences.
According to Meredith, the problem isn't just that people are short an hour of sleep. It's that even though we're getting dressed and going to meetings, our circadian rhythms are continuing to issue standard nighttime instructions regulating our heart rates, blood pressure, metabolic levels, and the release of digestive enzymes.
"Our activities get pushed forward before our ability to perform them has made the adjustment," she said.
That explains why even those people who try to compensate for the time shift by going to bed an hour early often have trouble nodding off. The same study that looked at workplace injuries found that on the first night of daylight saving time, people give up 40 minutes of sleep on average.
And as luck would have it, the type of sleep most likely to be sacrificed exacts the largest toll on our ability to concentrate.
As Kramer explained it, the body cycles through all the sleep phases every 90 minutes. But the first third of the night contains the largest proportion of deep sleep, while the final third has the most rapid eye movement sleep, the phase in which people dream.
"Deep sleep is when we restore our bodies," Kramer said."It's like a highway construction project, in which we send out chemicals that repair our joints and muscles.
"The last third of the night is given over to mind recovery. There's a theory that we throw out unneeded files to make room for the information we'll need to collect for the next day. For instance, it probably isn't important to remember the exact color of the tie your boss was wearing."
When the mental repair process gets short-circuited, Kramer said, people generally don't have a problem staying awake. They have a problem focusing.
"You may feel jittery and hyped-up," he said, "but you may find that you're not processing information quickly or accurately. That's just the way our brains are programmed."
But, that's not necessarily an excuse to call in sick on Monday.
For most healthy adults, it shouldn't take more than a day for our circadian rhythms to shift back by one hour -- though Kramer added that it might take as long as five days for insomniacs and other people sensitive to time changes to recalibrate their internal timekeepers.
Meredith suggested some practical measures that people can take to help synchronize their bodies and alarm clocks.
"The idea is to try to live as human beings used to live when we weren't divorced from the natural world," she said.
"Exercise, expose yourself to sunlight, and do it as early in the day as possible. If you have a choice between working out at the gym or working out outdoors, exercise outside. If you have a choice between riding your bicycle to work or walking over your lunch hour, ride your bike to work. Chances are you'll feel slightly more alert."
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