Though travelers can now zip through multiple time zones by simply steppingon a plane, their brains may work more slowly once they step off, warns a researcher at the University of Pittsburgh Medical Center. And those of us who don't leave home but do lose sleep may also think more slowly the next day—even if we feel wide awake.
Such effects aren't simply caused by fatigue, says Timothy Monk, Ph.D., but by compromising our bodies' circadian rhythms. When we work against these biological cycles of wakefulness and rest, our cognitive processes pay the price. Monk conducted a study, recently published in the journal Sleep, in which participants were kept awake for 36 hours and given hourly performance tests. Their response times increased at night, and also with every hour that they lost sleep.
"At night, our biological clock goes to work changing our body temperature, heart rate, hormones—changes that may make us think more slowly," he says. In the morning, an upswing in circadian rhythms creates a "second wind" that makes us feel more awake and energetic, even if our brains aren't up to speed.