Learning and Memory: How does sleep help?

Maria M. Hadjimarkou, PhD. Lecturer School of Psychology University of Sussex

We often give the advice to sleep well before an exam, or perhaps we have experienced a lack of sleep and found it hard to remember what we have previously learned. Does sleep really affect our ability to learn and remember? Yes, it can, and this is what I will be reviewing today.

Several studies set out to investigate whether the popular belief that sleep is important for learning and memory has any credibility. In the 1990s studies found that when animals go to sleep after learning a new task, like navigating through a maze, they do better at finding their way the next time if they are allowed to sleep. Recordings from the brain areas involved in this navigation showed that some neurons were active during exploration, but more excitingly, those same neurons were again active, in a similar way as in wakefulness, when the animals went to sleep. So, it became obvious that sleep probably helps us to learn new things by replaying what we have learned during the day and it helps to establish that new knowledge. This reactivation during sleep is thought to be part of a complex process that eventually consolidates or ‘fixes’ information so that it can be available for retrieval for a long time afterwards. Sleep deprivation hinders this process and leads to a worsening in memory and performance. Studies in humans have demonstrated that not only this is true, but there is more to appreciate. If you remember from the entry back in August, sleep is not a monotonous state where our brain shuts down until the next morning. It involves transitioning from one stage to the next, each stage involving different types of activity. It turns out that the different types of sleep serve different functions, which is kind of cool.

Slow wave sleep (SWS), the deep sleep which is characterised by slow activity which takes place mainly in the first part of the night, seems to support mostly learning that falls under the category of what we call ‘declarative’ or ‘explicit’. Declarative is information that we may need to learn and recall at will and we consciously bring that information to awareness, e.g. ‘Which is the capital of Denmark?’.
The other type of learning that we may need to learn falls under the category of ‘non-declarative’ or ‘implicit learning’ which includes procedural tasks such as playing an instrument, perfecting a movement for sport or dance, etc. These types of tasks which involve a sequence of steps and are not strictly recall of information, seem to benefit more from rapid-eye-movement (REM) sleep. REM takes place at the end of our first sleep cycle, about 90 minutes after we go to sleep and then re-occurs at the end of each cycle, with episodes lasting longer as we approach the morning hours. In the later part of the night, we tend to spend more time in REM sleep compared to the early part of the night. REM sleep has been strongly associated with dreaming even though now we know that dreaming can occur during all stages of sleep. You may have noticed waking up in the morning after being immersed in one of these dreams.

The fact that different types of learning are supported by different types of sleep, has implications regarding our sleep habits. Those who tend to procrastinate sleep and go to bed very late, are likely to be robbing themselves of the beneficial effects of SWS. Those who tend to get up very early in the morning and do not allow themselves to complete 7-8 hours of sleep, are likely robbing themselves of the beneficial effects of REM sleep on procedural and other types of non-declarative learning.

So, sleep helps to strengthen our learning and memory of things that we learned previously, and performance is better following a good night’s sleep. In some studies, there is evidence that performance in certain tasks will continue to improve with additional nights of sleep, so the processing of new learning may extend well beyond that first night after learning or a certain stage of sleep. Moreover, it seems that during sleep our brains make connections between things that are newly learned with previously processed information so that we may come up with insights or new revelations about things that we haven’t thought about before, or things that we have been struggling with. Pretty neat right?

But the story of sleep’s role in learning and memory does not end here. In addition to the fact that sleep helps to strengthen our learning, there is evidence that during SWS, another important process takes place: the process of deleting information that is deemed useless. Based on the effort and time that we spend on a given task during the day, our brain ‘decides’ on which aspects of our learning are important and thus should be kept and strengthened, in which case they are replayed and consolidated as mentioned above, whereas other aspects that are deemed not important are weakened. Thus, the strengthening of some information and the weakening or deleting of useless information is another important aspect of our ability to learn, and this important process seems to take place during SWS. So, sleep makes sure that our brains are not overloaded.

One major sleep researcher, Allan Rechtschaffen once said, “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made”. Well, evolution certainly did not make a mistake. Our brains are like mega-computers, that are always active, processing information. Our brains decide what is important based on our experiences and store the important bits, delete the unimportant bits and work non-stop to find meaning. Luckily, we don’t have to sit still or stay focused 24/7 in order to help our brain do all these things. We can just go to sleep and let it handle it. We can just relax and hand over control, which is liberating. We are taken care of.

Key Reference: Stickgold R. (2005). Sleep-dependent memory consolidation. Nature 437, 1272–1278.