Ask a Neuroscientist: On Sleep and Memory

Author's note: Read this post in Dutch!

I’ve always wondered if there is any connection between memory and sleep? Do people who sleep well do well in Exams? Is 5 hrs of sleep sufficient for a college going student in todays tight schedule?
— Akshay Sekar

Sleep and memory, these terms have become intricately intertwined over the past decades. Common beliefs are that a good night’s sleep will enhance recall of old and learning of new memories, prevent memories from decaying, and improve insight. But how much of these premises are actually true? Does the right amount of sleep really work miracles when it comes to learning and remembering information? And if so, how can we use these insights in school?

These questions seem simple at first sight, but they are still puzzling many researchers on a daily basis. Below I will point out a few reasons why the relation between sleep and memory is not as straightforward as one might think.

Neither sleep nor memory is unitary

 Unfortunately, both sleep and memory are more complex processes than they appear on the surface. This non-unitary nature makes it harder to generally state that sleep affects memory. Rather, it seems that certain types of sleep affect certain types of memory.

 You might not notice, but you sleep in stages. Our brain systematically cycles through these stages between the moment we hit the pillow, and the alarm clock ringing. Researchers dissociate two main sleep stages: non-REM and REM-sleep. During Rapid Eye Movement (REM) sleep, as the name implies, your eyes move all over the place. Also, your brain waves resemble awake brain waves, and you are very likely to be dreaming.

 Non-REM sleep is a bit more boring in terms of eye movement and brain waves, but not less interesting for researchers. Non-REM sleep is subdivided in 4 stages, each corresponding to slower brain waves. The final stages (stage 3 and 4) are also called deep sleep or slow-wave sleep and are, next to REM-sleep the favorite sleep stage for memory researchers. And yes, whatever the brain is doing during each of these stages, it seems to relate to memory in some way.

Just as sleep is not unitary, neither is memory. There are many different types of memories: from the memory of your last holiday to the memory of how to ride a bicycle. For the sake of simplicity I will just focus on one distinction within the vastly complex memory system in our brain, that of declaration. This distinction became apparent after researchers encountered epilepsy patient Henry Molaison, who was unable to construct new memories after an operation, but was able to learn new movements. Based on these observations, the brain’s memory system was divided up into declarative memories (memories that can be declared, such as the memory of your last holiday) and non-declarative memories (memories that cannot be declared, such a knowing how to ride a bicycle). These types of memories appear to depend on different parts of the brain and are thus assumed to be relatively independent of each other.

Memories are not sole entities; they like to merge with other, previously stored memories. This means that details about what you have learned will become more blurry over time (and sleep). Researchers think this happens through consolidation of memories, a process that is thought to stabilize and integrate memory traces, often at the expense of specific memory details. This merging process is likely useful because a network of old memories can help store new memories more easily, thereby speeding up learning of overlapping information.

Sleep and memory interact

 So given their non-unitary nature, how are sleep and memory connected? As I touched upon above, researchers are now finding an apparent relation between certain sleep stages and effects on different types of memory. To put it boldly: REM-sleep appears to benefit non-declarative memories whereas slow-wave sleep leads to enhanced declarative memories. Even though the underlying mechanisms are still mostly unclear, this seems to be the general picture over multiple experiments. Now, does this mean that getting a lot of slow-wave sleep before an exam is the best thing to do when you want to nail it? Unfortunately this is not the whole story; there are more factors in play.

To illustrate this, think back to your last holiday. If this last holiday was a few weeks ago, you will have more detailed memories about it than when it was a few months ago. We appear to forget more than we can remember, especially if we are not reminded regularly or have little cues to guide recall. This gradual forgetting means that consolidation experiments very rarely yield memory enhancement over sleep. Rather, experiments on the effects of sleep on memory examine a less strong decrease in memory than can be expected due to mere forgetting over time.

Time passed and sleep are necessarily related

 The unequivocal relationship between forgetting, consolidation and time puts a strong burden on sleep research because sleep necessarily takes time while time without sleep adds interference.  Researchers try to circumvent this by adding non-sleeping control groups that are tested with the same time difference between learning and testing. This means the control group either needs to be tested on a different time of day or has to be kept awake while the other group goes to sleep, and this brings about a some issues.

 Anyone who pulls an occasional all-nighter knows that not sleeping can severely affect your attention span, motivation, and focus. Unfortunately, these things are crucial to your memory performance as well. A control group that has not slept is thus a poor control group. So researchers have started to either allowing only 4 hours of sleep, or specifically target certain sleep stages, waking the participant up or trying to reactivate a memory when a certain sleep stage is reached (which can be measured using EEG). This practice means that when an experiment concludes that memory is better after a night of sleep, you should always consider this is relative, compared to a non-sleeping or poor-sleeping control group. 

So, what DO we know?

I hope I made clear that the question “does sleep benefit memory” is hard to answer because of the strong dependence on the type of sleep you experience and the type of memory you want to access. Furthermore, most sleep and memory studies compare a full night’s sleep to suboptimal or no sleeping, making it hard to draw conclusions about whether sleep is truly beneficial to memory, rather than just harmful when absent.

Nevertheless, researchers are now pretty confident that sleep (especially slow-wave sleep) affects declarative memory, particularly integrative features such as getting the gist or making associations. This does often happen at the expense of specific details that tend to fade over consolidation during sleep. So if you carefully want to build your knowledge over time, as is important for many school subjects, you’d better sleep consistently but keep track of fading details as well!

Some tips

Ok, so we unfortunately still know relatively little about the interactions between sleep and memory. But what can a neuroscientist like me advice you about the importance of sleep for performance in school settings? Let’s see:

  • Get into a rhythm so you wake up naturally and not from your alarm clock. Try to get the recommended 7-9 hours of sleep so that you can cycle through all the sleep stages, as all of them appear to have a unique and essential function. If you haven’t gotten enough sleep, daytime naps and wakeful rest are also suggested to benefit learning and memory.
  • Yes, cramming and pulling all-nighters will probably help you nail an exam on the next day. However, you will have forgotten the information soon after. Studying in little bits during a longer period of time (including sleep) helps you build up knowledge structures that stick around longer and aid integration of new information.
  • Because details blur over time, it is smart to repeat some specific details such as definitions, mathematical functions, or dates on the night before an exam. If you have studied the material consistently, you shouldn’t have to worry too much about the big lines, but will benefit from going over these details at the last moment! However, do not forget to sleep afterwards to be able to focus during your exam.
  • Take your time. A strong memory and deep understanding takes a while, and here the beneficial role of sleep is clearly present. For example, if I have to struggle through a complex paper or have to read into a new research area, I read bits and pieces over multiple days or even weeks until I notice the big picture emerging. It greatly helps me to divide such tasks up and revisit previously learned things regularly. And, most important of all: information appears to stick around longer if you give it some time. So take the time if possible, you won’t regret it!