When you work late at night, you probably know you’re feeling more and more tired. But researchers have found that your actions may get slower and slower, even if you don’t realize what’s happening.

Researchers in Brigham and Women’s Hospital in Boston have found that lack of sleep can affect how you perform certain tasks, regardless of how tired you think you are. The findings were published last week in the online Journal of Vision.

The one-month study looked at how people perform complex visual search tasks, which are part of many safety-sensitive activities, like air-traffic control, baggage screening and monitoring industrial plant operations. This kind of work, which often must take place late at night, involves quickly and repeatedly encoding and retrieving visual information, then using it to make decisions.

After a preliminary week in which participants slept 10-12 hours a night to be sure they were well rested, the next three weeks had participants sleeping about 5.6 hours per night, following a 28-hour cycle to duplicate the effect of chronic jet lag. Researchers asked participants to perform visual search tasks and recorded how quickly and accurately they were able to find and identify visual information.

As study participants stayed awake longer, they identified information more and more slowly. If they were working from midnight to 6 a.m., participants were slower than they were during the day. As the weeks went on, participants’ accuracy continued to be fairly consistent, but they slowed down further. Their self-ratings of sleepiness declined only slightly, but the data showed that they were significantly slower.

While it may seem obvious that being sleepy slows you down, this research has significant implications for employers and workers. The lessons are that people who do this type of work during the night shift will be significantly slower. The later they work the slower they will be, and they may not realize they are impaired.

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