Two new reports have underscored the contribution of a good night's sleep to forming and retaining memories.
In the first, researchers at Harvard Medical School used a single 60-90 minute training session to instruct 133 individuals on how to perform a visual discrimination task. The participants were retested on a second occasion three hours to seven days after training.
Those who were able to sleep between learning and retesting showed marked improvement. But those who were retested within three hours of the training session and who therefore did not have the benefit of rest after the initial learning session performed worse.
Several of the study participants were deprived of sleep from the time of training until nine o'clock the following evening. They were then allowed to sleep as much as they liked on the second and third nights. These participants also did not perform well in the discrimination task.
The results suggest that sleep has a vital, but unexplained, role in the process of assimilating and retaining information (Nat Neurosci, 2000; 3: 1237-8).
In the second study, German researchers compared the effects of 'early' or slow wave sleep (SWS) and 'late' or rapid eye movement (REM) sleep, on the same visual discrimination task. After the study participants were trained to do the task in the evening, they were retested after three hours of sleep early in the night. It was found that their performance improved.
In contrast, individuals trained in the middle of the night, who then rested for three hours, did not improve.
The researchers suggest that a two step process is involved in making a memory during sleep which is initiated during SWS sleep and consolidated during REM sleep (Nat Neurosci, 2000; 3: 1335-9).