If you seem to have trouble thinking straight late at night, don't put all the blame on long hours of work or lack of sleep. According to a new study, your brain simply works slower in the wee hours. Published this week in the journal Sleep, the research suggests that the human biological clock slows down the brain's performance at night, exacerbating the mental wanderings that come from lack of sleep.
Most previous studies of human performance without sleep have focused on sleep deprivation. But Timothy Monk and Julie Carrier from the University of Pittsburgh wanted to find out whether circadian rhythms--the daily cycles that tell us when to wake and when to sleep--also affect the brain's information processing.
The researchers kept 18 participants awake in rooms without windows or clocks for 36 hours, and to remove the soporific effect of large meals, they gave the subjects hourly nutritional supplements. The subjects were tested every other hour with true-false questions phrased in both positive and negative voices, such as "C is before M--MC" and "M is not before C--MC." Although the negative-voiced questions always took longer to answer, the difference in answer times between the two question types yielded a "relatively pure measure of speed of thought" at different times throughout the study, says Monk. Any lack of attention or slowness in reactions would on average cause the same delays for both types of questions.
Although the subjects became progressively slower at the harder questions as the study wore on, they improved slightly the morning after the sleepless night. Monk and Carrier attribute this burst of sharper thinking to circadian rhythms, as the subjects' biological clocks were still active.
The study shows that it's not just sleep that night workers have to fight, says human chronobiologist Scott Campbell of Cornell University: "Just by running cold water on your arms and putting a cold rag on your head to try to make yourself feel transiently alert probably is not going to bring your performance back to daytime levels." But he cautions that the study isn't perfect: "It's still difficult, even in this protocol, to tease out the difference between sleep deprivation and the true circadian, time-of-day effect on performance."