[This is our synchroblogging post in honor of PLoS ONE's second birthday. Why not write your own?]
Ever wonder whether it’s better to study all night before a big exam, or to get a good night’s sleep, but maybe not have a chance to go over all the material? We know that memory consolidation can occur in sleep, but we also know that those extra hours studying can do some good. And then there’s the issue of false memories: who hasn’t had the experience of being completely convinced their test answer is correct, only to learn that it’s 100 percent wrong?
Inducing false memories is surprisingly easy. All you have to do is present people with a list of words to be memorized, like this:
Then, 12 hours later, you test them: Was “backpack” on the list? How about “television”? “Mountain”? “Trapeze”? “Hike”?
It’s easy to remember the items that were on the list, especially considering they all relate to a common theme. It’s also easy to eliminate items like “television” and “trapeze.” But what about “hike”? That was the theme of the list, but it wasn’t on the list. Most people will falsely remember that it was, especially if they had to memorize several similar lists all at once. And they’ll be just as convinced that it really was on the list as any of the other items.
So how are these false memories formed? We know that memories are often consolidated in sleep, so the question arises: how does sleep affect false memories?
A team led by Susanne Diekelmann gave volunteers 18 such lists, each 15 words long, then systematically deprived some of them of sleep. Some were given the words at night (played via an audio recording), then required to stay awake until morning in the lab. Some were allowed a normal night’s sleep. And some were given the list in the morning after a normal sleep. All were retested nine hours later. Here are the results:
As you can see, everyone had plenty of false memories, but those who stayed up all night had significantly more. People who had a good night’s sleep were twice as likely to avoid false memories as those who stayed up all night. Everyone was just as accurate on normal memories — only false memories were affected in this way.
Then the researchers repeated the experiment over two nights. Everyone was given the words to memorize on the evening of the first day and tested on the morning of the third day (following the second night) This time, some participants were deprived of sleep on the first night, and some were deprived on the second night. Here are the results:
Only those who were deprived of sleep during the second night had a significantly higher false memory rate than those who slept both nights — sleeping during night 1 had no effect on the results. So it seems that exhaustion at the time of testing may be responsible for the results, not memory consolidation during sleep.
In a final experiment, the words to memorize were given in the evening and everyone was deprived of sleep the following night and tested in the morning. But one group of volunteers was given caffeine an hour before testing. The other group was given a placebo. Here are the results:
So caffeine wipes out the false memories! The placebo group was indistinguishable from the night-wake group in the first experiment.
But shouldn’t the people who didn’t get a good night’s sleep immediately after memorizing the words have had trouble consolidating those memories, and thus performed worse than those who slept through the night? The researchers say that sleep-consolidation isn’t as big a factor for recognition memory, as in this test, compared to other types of memory, such as free recall. It would be interesting to see this study repeated with a free recall test. Millions of last-minute students studying for exams want to know.
One thing’s for sure: if you do stay up all night studying for an exam, make sure you caffeinate yourself before taking the test!
Susanne Diekelmann, Hans-Peter Landolt, Olaf Lahl, Jan Born, Ullrich Wagner (2008). Sleep Loss Produces False Memories PLoS ONE, 3 (10) DOI: 10.1371/journal.pone.0003512