Coffee can’t fix all the cognitive impairments caused by a bad night’s sleep

By Emma Young

Many of us “treat” a bad night’s sleep with a couple of cups of coffee the next morning. But just how much does caffeine really help? Research shows that it makes a big difference to tasks that require “vigilant attention”, or continuous monitoring. One theory even holds that the cognitive deficits caused by sleep deprivation are underpinned by impairments in attention, and this implies that caffeine could be a general cure for sleep-deprivation ills. However, a new study in the Journal of Experimental Psychology: Learning, Memory and Cognition suggests that this is not the case. Caffeine indeed restored vigilant attention to regular levels in sleep-deprived participants. But it had barely any impact on another type of performance that is important for all kinds of jobs.

Michelle E. Stepan at the University of Pittsburgh and colleagues studied 276 students with normal sleep habits. They came into the lab in the evening to complete two tasks. The first was a simple computer-based visual vigilance task — a red circle appeared on the screen at random intervals and every time they saw it, they had to click a mouse. If they took longer than 500ms to respond to a circle, this was counted as a lapse in attention.

The second task measured what’s called “place-keeping”. In this study, the participants had to keep track of where they were in a seven-step screen-based task. The participants were initially trained in how to complete the steps correctly, in the right order, and then expected to remember this information. Every so often, they were interrupted part-way through a sequence and asked to complete another brief task. They then went back to the original task. To resume correctly, and complete the steps in the right order, they had to remember where they’d broken off. The researchers counted the number of mistakes that they made.

After this, some participants were sent home to sleep as normal while the others were kept awake overnight in the lab. The next morning, the sleepers came back in, and all were given a capsule that contained either 200mg of caffeine or a placebo. Then they completed a second round of visual vigilance and place-keeping tests.

The differences in the results were clear. Sleep deprivation (without caffeine) impaired performance on both tasks, as expected. With caffeine, though, sleep-deprived people did just as well on the visual vigilance task as those who’d had a good night’s sleep (without morning caffeine). Caffeine clearly worked very well to fix this particular deficit. (Though those who’d slept and consumed caffeine did best of all.)

However, this wasn’t the case for the place-keeping task. A few did get some benefit from caffeine — the people who were worst at the task. But most of the sleep-deprived participants did not. For the majority, caffeine did not affect the cognitive processes needed for place-keeping.

The work implies that the theory that all of the cognitive deficits associated with sleep deprivation can be explained by impaired attention is wrong. And the team now calls for more work to get at exactly how a lack of sleep interferes with various types of cognitive processes.

But there’s an important practical message, too. Remembering where you are in a sequence of key steps, so that you can complete a procedure properly, is important in jobs everywhere from factories to hospitals. While caffeine does indeed help with simple attention, it could be a potentially dangerous mistake to think that it can fix sleep-deprivation deficits in performing this type of task, too.

Caffeine selectively mitigates cognitive deficits caused by sleep deprivation.

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest