ATLANTA, GEORGIA--Going a night without sleep may cause your hippocampus to go on strike. A new study has caught this crucial memory-encoding brain region slacking off in college students the day after they've pulled an all-nighter. The study is one of the first to investigate how sleep deprivation interferes with memory mechanisms in the human brain.
Neuroscientist Matthew Walker of Harvard University and his colleagues paid 10 undergraduate students to forgo a night's sleep. The next day, the students viewed a series of 30 words, and two days later--after having two nights to catch up on their sleep--the students returned to the lab and took a test to see how well they remembered the words they'd seen.
The students recalled about 40% fewer words overall than a group of 10 students who had slept normally, Walker reported here yesterday at the annual meeting of the Society for Neuroscience. But the researchers also found that the emotional content of the words made a big difference in what people remembered. Previous studies have found that both positive and negative emotions bolster memory, but in the current study, negatively charged words (such as cancer or jail) seemed to penetrate the sleep-deprived brain more deeply than positive ones (such as happy or sunshine). Indeed, sleep-deprived students were only 19% worse than their well-rested counterparts at remembering negative words, but 59% worse for positive words. Walker suspects the difference may reflect an evolutionary safeguard against forgetting potential threats.
To find out which part of the brain was responsible for this forgetfulness, the researchers repeated the experiment with a different group of undergrads, but this time used functional magnetic resonance imaging (fMRI) to monitor brain activity while the students viewed a set of emotionally neutral photographs. The fMRI scans revealed lower activity in the hippocampus of sleep-deprived students than in well-rested students. This suggests that just as sleep is important for consolidating new memories after they're learned, as other studies have shown, it's equally important for preparing the brain to learn new things the following day, Walker says. His team is now using fMRI to investigate the emotional memory bias.
The work raises many interesting questions about the relationships between sleep, memory, and emotion, says William Fishbein, a neuroscientist at the City College of New York. He adds that the findings are consistent with studies with rodents done in the 1970s that suggested that sleep deprivation prior to learning prevents new memories from sticking.