Sleep On It

Art by Samuel Pilgrim.

We spend approximately a third of our lives sleeping. Why do we do it? What is it for? Sleeping animals are very vulnerable, so evolution would surely have selected against sleep if it did not serve some important purpose. Nevertheless, the reason why we sleep has remained elusive for many years.

Recent research suggests that a major role of sleep is to consolidate memory; newly-acquired memories are strengthened and integrated with previous experience while we sleep.

As we go about activities in our daily lives, such as talking to a friend in a cafe, our brain processes the sensory ingredients that make up our experience: the content of the conversation, the smell of coffee, the sound of the cash register and so on. This processing takes place in various specialised regions in the outermost layer of the brain, called the neocortex. The separate sensory elements are rapidly bound together in an area deep inside the brain called the hippocampus. During subsequent sleep, the memory in the hippocampus is repeatedly reactivated, triggering reactivation of our memory in the neocortex. This process not only strengthens the memory, but also reorganises the way it is stored in the brain, allowing it to be integrated with existing memories. As a result, the information is stored more efficiently and can be used more flexibly; we are better equipped to identify information that is consistent across episodes (that your friend has developed a habit of complaining about their boss, for instance) and to make inferences (that your friend, who has a history of criticising people they are actually attracted to, may harbour a secret crush on their boss, for example).

This process of ‘reactivation’ is thought to happen during a specific stage of sleep called slow-wave sleep (SWS). There is a pattern of brain activity prevalent in SWS that drives reactivations in the hippocampus. Indeed, researchers have shown that artificially enhancing the slow-wave using brain stimulation improved their participants’ memories on the information learnt the night before.

Reactivation

Some evidence for the reactivation of memories during sleep comes from brain imaging studies. In one experiment, a group of people learnt how to navigate through a virtual reality environment and then slept in a brain scanner. It was found that the hippocampus was more active in subsequent SWS for these individuals than for people who had not been trained on the task. Thus, the hippocampus was ‘reactivated’ in SWS. Most importantly, the amount of hippocampal reactivation in the trained subjects was positively correlated with overnight improvement on the task.

Observations of reactivations in humans are based on the activity of a whole brain region. However, in rats it is possible to record the activity of individual neurons. This technique has provided much stronger evidence that the actual patterns of neuronal activity associated with waking behaviour are replayed during subsequent SWS.

To be sure that such reactivations actually benefit our memory, researchers must manipulate them rather than just passively record them. Published in the prestigious journal Science in 2007, Rasch and Born at the University of Lübeck, Germany pioneered the first of a series of experiments that did just that. Their subjects learnt the locations of 15 card pairs on a computer screen in the context of the smell of a rose. Re-presentation of the odour during subsequent SWS led to reactivation of the hippocampus and improved memory for the card locations. However, you can’t just whip out some potpourri at bedtime the night before a test and expect to boost your score; Born and colleagues confirmed that the smell must also be present during learning to have this effect. Interestingly, re-presentation of the smell during wake and other stages of sleep neither benefited memory nor reactivated the hippocampus. This study suggests that when reactivations happen during sleep they consolidate our memory. It also highlights the possibility that during SWS, the hippocampus is especially sensitive to stimuli capable of inducing reactivations.

So, you may learn something new every day, but every night helps you remember it!

Reorganisation

While you are sleeping, your brain makes sense of your waking experiences.
A recent experiment demonstrated that sleep can help you to gain insight. People were asked to process strings of digits—there was a hidden structure to the strings that meant that the answers could be reached much more quickly using a simple rule, but the participants did not gain insight into this rule during the initial training period. More than twice as many participants gained insight into the rule after eight hours of sleep compared to after equivalent periods of nocturnal or day-time wakefulness. This suggests that the gain must be a consequence of the sleeping brain re-processing information learnt whilst awake.

Sleep can also help us make connections. Last year, Fischbein and coworkers at City College of the City University of New York taught people to associate each of thirty objects with two different sets of faces. In a subsequent test, it was found that the subjects who had been allowed to nap after learning were better at identifying which faces were associated with one another, as they shared a link to the same object. This so-called ‘relational’ memory performance was correlated with the amount of SWS during the nap. This illustrates the power of the sleeping brain to integrate memories, allowing us to associate things that we have never actually seen together.

Practical Applications

How much sleep should we get and when?

Interestingly, you do not need to get a decent night’s sleep to consolidate your memory. In fact, some studies have shown that a nap of one to two hours can be just as good for your memory as a full night of sleep.

As to when you should sleep, the time of day does not seem to matter. Studies have shown that nighttime sleep is no better than daytime sleep for consolidation of skill learning. Furthermore, researchers have found that memories for verbal material (e.g. nonsense-syllables, words and stories) are equally affected by morning and evening naps.

However, when your sleep falls relative to your waking activities is important. For instance, if you have an assignment to complete, it is a good idea to sleep between the reading and writing stages. This way, you can let your sleeping brain do some of your thinking for you. You may wake up with new insights and a ready-made essay structure!

Eau de memory

If you have something particularly important to learn, you would be well advised to invest in a timed-release air freshener. As in Born’s experiment, turn the air freshener on while you study. Then set the device to pump the smell into the room again while you are napping. Your brain will retrieve the associated information and strengthen your memory.

To ensure that you don’t habituate to the smell, it should be delivered in pulses rather than released constantly. Don’t use a commonplace smell and don’t re-use it; this will reduce the specificity of the association and so dilute its power as a reminder.

While many mysteries about sleep remain, it is now clear that replacing sleep with work is unwise if you want to remember the information in the long run.
And, don’t forget, if you’re struggling with a problem, try sleeping on it!

Kathryn Atherton is a 1st year DPhil student in Neuroscience at New College.

Art by Inez Januszczak.