Humans have been fighting jet lag for some 50 years. In that time we've cast far and wide for tricks to rejigger sleep cycles disrupted by airplanes. There are dubious herbal supplements and, more dubiously, earbuds that blast light toward the brain. Sleep experts suggest soaking up finely portioned amounts of sunlight, with more light early in the east and waiting till later in the west. For the desperate, a perky coffee or sleep-inducing beer can be a tempting - although not always advisable - option.
One folk observation, however, seems to ring true. As frequent cross-continental travelers might already suspect, it is generally tougher to adjust when traveling east. Go west, on the other hand, and rest a little easier.
That's because our sleep cycles need different amounts of time to readjust. It is easier, say scientists at the University of Maryland, for our neurons to cope with a prolonged day than a shortened one. Traveling west - backward in time zones - adds hours to the day, closer to the longer day most human bodies naturally prefer.
Our bodies' sleep cycles are ruled by neurons that act like pacemakers. The individual neurons normally work in concert to make us feel alert in the morning and awake at night. But for those neurons to stay synchronized, we must be exposed to light, akin to winding a slow clock. When we travel, that tuning period falls apart.
Writing in the journal Chaos on Tuesday, the researchers mathematically modeled brain cells that control sleep, to better understand why traveling to eastern time zones may produce rougher bouts of jet lag.
To show this, the scientists created a simplified model of the area of the brain called the suprachiasmatic nucleus. This is the cluster of neurons that regulates the circadian rhythm. That rhythm, or internal clock, kicks us awake and will later tell us it's time to sleep. Each of us has rhythm that repeats roughly every 24 hours, with an ebb and flow of sleep hormones.
Crucially, circadian rhythms are not perfectly attuned with the human ideal of exactly 24 hours. On average, circadian rhythms last a little bit longer - about 24.5 hours.
"Our model suggests that the difference between a person's natural period and 24 hours controls how they experience jet lag," Michelle Girvan, a physics professor at the University of Maryland, said in a statement.
In the study, Girvan and her colleagues sent fictional travelers on trips around the world, crossing increasing numbers of time zones. Those average extra 30 minutes on the clock seem to make a large difference, they found, in making western trips easier on the brain.
The scientists mapped out how long it would take for their model brains to recover from increasingly distant trips. That is, their model was a synthesis of all the neurons that individually spin through a daily cycle. For people suffering from jet lag, the neurons pop out of sync. The result is like a row of watches all showing slightly different times. As we recover, the neurons start to show the same time again.
Traveling west over six time zones requires an average of about six days to fully recover, based on the model, whereas traveling east jumps up to eight days. At nine time zones crossed, the difference is even starker: about eight days to recover from a western trip, but 12 days from an eastbound one.
This study is supported by earlier research into what we know about the interplay between light and the circadian rhythm, as light is the main mechanism that rewinds the biological clock. Our anatomy reflects this, too. The nerve fibers that link the eyes and brain thread right below the suprachiasmatic nucleus.
When travelers head east, hours are lopped off from the day - and the amount of sunlight available - which means we want to stay awake longer in the dark. Conversely, heading west prolongs the day, which may better complement our internal clocks.
Importantly, not all internal clocks tick at the same rate. That varies, explained Girvan, depending on the person."Some people may have a natural circadian rhythm with a period of 24.5 hours," she said, "while others may have longer or shorter natural rhythms." Variations among our internal clocks are thought to be why jet lag effects some people more than others.
Girvan hopes this research can lead to better fixes for out-of-whack circadian rhythms, whether they arise from "rapid cross-time-zone travel, shift work, or blindness." Presumably, none of those treatments involve shining light down people's ear canals.
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