Researchers from Brigham and Women's Hospital, Harvard Medical School and the University of Pennsylvania found that sleep benefits an individual's ability to recall recently learned declarative memories, even when recall of these memories is challenged hours later by competing information.
This finding is particularly important for individuals with mentally demanding lifestyles, such as doctors, medical residents and college students, who often do not get adequate amounts of sleep.
The study appears in the July 11, 2006 issue of Current Biology.
Declarative memories, or hippocampus-mediated memories, are types of memories about facts and events that can generally be put into words. These differ from procedural memories, or nondeclarative memories, that are "how to" memories, which have been studied and shown to benefit from periods of sleep. However, until this point, no one has been able to definitively demonstrate that sleep promotes the strengthening of declarative memories in humans.
Sixty participants (33 women and 27 men) between the ages of 18 and 39, who did not use prescription or illicit drugs and did not have known sleep disorders or abnormal sleep patters, were chosen for the study. Forty-eight individuals were assigned to one of four groups: sleep before testing, wake before testing, sleep before testing with interference, or wake before testing with interference. All groups learned 20 paired words, which had no obvious semantic relationships. Twelve hours later, everyone was tested for recall by completing a cued-recall task; however, individuals within the interference groups learned a second list of 20 word pairs before testing. Participants were prevented from rehearsing the new words after learning the new list. To address the concern for time-of-day effect, the researchers ran an additional, independent group (12 individuals): 24-hour p.m. to p.m. with interference and sleep or wakefulness.
The researchers found that in the non-interference groups mean recall was slightly higher in the sleep group compared to the wake group; however, participants in the interference condition who were able to sleep did significantly better on the recall task than did the wake group. A circadian confound was excluded.
"The results show that the sleep benefit for memory persists across the subsequent waking day, even when challenged by new information," said lead author Dr. Jeffrey Ellenbogen, an associate neurologist at Brigham and Women's Hospital and postdoctoral fellow in sleep medicine at Harvard Medical School. "This shows us that sleep does not just passively protect memories, but rather, plays an active role in memory consolidation."
This is an especially important finding for our society, which attempts to replace sleep to improve efficiency, explained Ellenbogen. "You can take a drug to stop hunger pains, but eventually you will need to eat food for vital body functions. The same goes for sleep. Taking a stimulant to stay awake can help with alertness in the short term, but people really need sleep to retain knowledge and function at their best."
Ellenbogen hopes this finding will alert the academic community to the critical benefit sleep plays in learning, which is of particular importance to sleep-deprived medical residents and college students notorious for "cramming" the night before an exam. He hopes further research will look into improving the quality of sleep for individuals with sleep disorders and for aging populations who often have impaired sleep quality and memory loss.
Co-authors of the study include Justin C. Hulbert, Robert Stickgold, David F. Dinges and Sharon L. Thompson-Schill.
The National Institute of Health and the University of Pennsylvania's Nassau Undergraduate Research Fund supported the research.