When I was a child and I had to learn something, I was always encouraged to read things one last time before I went to sleep. Now there is some research to indicate that this was excellent advice.
We have known for decades that sleep helps improve memory for procedural tasks, such as learning a new piece on the piano. But nobody has known for sure whether sleep helps other types of memory.
A study that presented at the 59th Annual Meeting of the American Academy of Neurology (AAN) in Boston, Massachusetts showed that declarative memories — memories for facts and events in time — become more resistant to interference during sleep.
To test whether sleep strengthens declarative memory in the face of interference, a research team led by Dr. Jeffrey Ellenbogen from Harvard conducted a study in which 48 people between the ages of 18 and 30 were divided into 4 groups:
- A wake group without interference
- A wake group with interference
- A sleep group without interference
- A sleep group with interference
Each group was taught the same 20 pairs of words in the initial training session. The wake groups were taught the word pairings at 9 AM and then tested on them at 9 PM, after 12 hours of being awake. The sleep groups were taught the word pairs at 9 PM and tested on them at 9 AM, after a night of sleep.
Just before testing, the interference groups were given a second list of word pairs to remember. The first word in each pair was the same on both lists, but the second word was different. The idea was that this would test the brain's ability to handle interference. The interference groups were then tested on both lists.
The investigators found that subjects in the sleep groups had superior recall, when compared to those in the wake groups. The difference between the sleep and wake groups was greatest when the subjects were tested after interference (76% vs 32% of words recalled correctly in the sleep group vs the wake group. This result was statistically impressive: P < .0001).
This is important research: not only does it help us advise people on how and when to study, but it will likely have important implications for our understanding about how memories are laid down and encoded in the brain.