Allan Rachtschaffen, a pioneer in sleep research, once said that if sleep did not serve a vital function then it would be the biggest evolutionary mistake ever made. He was probably right. Research for the past two decades or so has shown that a good night´s sleep is important for consolidating new learning in long-term memory. And this is not only because sleep restores energy and therefore makes us more able to learn new things. Sleep actually has a direct effect on memory consolidation. Our need for sleep should therefore not be underestimated!
Slow-wave sleep and memory consolidation
Most people are familiar with the fact that while sleeping we pass through several cycles. Our sleep during these cycles can be broadly divided into two types; non-rapid eye movement (NREM) sleep of various depths and rapid eye movement (REM) sleep, during which we dream. During the deepest stages of NREM sleep the brain´s electrical activity (EEG) is dominated by slow-wave activity. Therefore, we can refer to these sleep stages as slow-wave sleep (SWS).
When we learn new things the information is learned, or encoded, and temporarily stored by the hippocampus and surrounding areas in the temporal lobes. The hippocampus also connects with the areas in the cerebral cortex (neocortex) that were active during learning. During subsequent slow-wave sleep (SWS) this hippocampal-cortical neuronal network is reactivated and the cortico-cortical connections are strengthened. Ultimately the information is redistributed to neocortical areas that are by now connected in a network and take over the storage of the consolidated long-term declarative memories. However, the hippocampus is probably not entirely off-the hook as its involvement may be required for later recall.
It is fascinating that SWS is critical for the consolidation of declarative memories. It is natural to associate learning with an active process. Yet, it turns out that being unconscious, or “off-line”, for quite a while every night is critical for a healthy memory! So if you regularly pull an all-nighter cramming for an exam you should consider taking up other study habits.
Sleep-deprivation prior to learning
Sleep-deprivation prior to learning also impairs memory because without adequate sleep the temporal lobes of the brain are unable to fully engage during learning. Furthermore, sleep deprivation may cause us to focus more on memories with a negative emotional tone thus biasing what we remember. This might partly explain how lack of sleep could lead to depression.
Make sure to sleep enough!
Have you ever heard someone proudly say: “Oh, I only need four hours of sleep”? Margaret Thatcher was one of the lucky few; she apparently only slept four hours a night. But we are not all this fortunate. Most of us need to sleep a lot more, or 7-8 hours on average.
In my view sleep is under-valued and we should not take pride in sleeping too little. Respect individual differences and enjoy your sleep! Sleep as much as you need in order not to be sleepy and tired during the day. At the same time you will boost your memory.
If you have problems sleeping maybe the advice here can help http://www.sleepfoundation.org/article/ask-the-expert/sleep-hygiene
Listen to the following NPR story on sleep and memory: http://www.npr.org/templates/story/story.php?storyId=120573613
Born, J. & Wilhelm, I. (2012). System consolidation of memory during sleep. Psychological Research, 76, 192-203.
Marshall, L., Helgadóttir, H., Mölle, M. & Born, J. (2006). Boosting slow oscillations during sleep potentiates memory. Nature, 444(7119), 610-613.
Walker, M. P. (2009). The role of sleep in cognition and emotion. The Year in Cognitive Neuroscience 2009: Annals of the New York Academy of Sciences, 1156, 168-197.