Deep sleep found to be important for memory function

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Variously on this site I have extolled the virtues of sleep. Only last week I reported on a study which showed that shorter sleep duration and lower sleep efficiency (percentage time spent asleep as a percentage of time spent in bed) were associated with significantly advanced risk of the exposure to the cold virus leading to a full-blown cold (see here). Much of the sleep research, such as this latest study, has focused on the relationship between health and quantity of sleep. However, what about sleep quality?

Sleep comes in varying depths, and the deeper it is, the slower brain waves become. Shallow sleep (a situation in which entry to deep, slow wave sleep is limited) can be induced by a number of factors including sleep apnoea (short periods of arrested breathing during sleep, often caused by excess weight), as well as light and noise pollution. It’s also more common as individuals age. In an a recent study, shallow sleep was induced in individuals with an average age of 60 to see what effect this had on their brain function, specifically their memories.

In this test the individuals were shown 50 pictures and, following a normal night’s sleep, were later that day shown 100 pictures containing the 50 pictures they had been shown the day before. Their job was to pick out the pictures they had seen. The test was repeated at another time (using different pictures), but this time individuals were asked to identify previously seen images after a night of ‘shallow sleep’. Here, individuals were exposed to a beeping sound that did not impair length of sleep, but is designed to impair deep sleep. Total sleep time in each of the tests. In other words, the variable here wasn’t how long individuals slept for, but the depth of the sleep.

This study found that performance was significantly impaired by short sleep. Specialised brain scanning (functional MRI) showed that after shallow sleep, individuals had diminished blood supply to part of the hippocampus on the right side of the head (the hippocampus is part of the brain involved in memory). What this study suggests is that it’s not just getting enough sleep that is important for optimal memory, but getting enough deep sleep that counts.

Two simply measures that might help here concern light and noise. Ideally, bedrooms should be as dark and quiet as possible. However, it’s not always easy to assure such an environment, even in our own home. And for frequent travellers, it’s even harder.

A couple of cheap and simple things that might help here are earplugs and eyeshades. I’ve been using earplugs (the foam sort) for years, primarily when I travel. I’ve found them invaluable on planes and for dulling extraneous noise in hotel rooms such as traffic and corridor noise, humming mini-bars and whirring air conditioning units. I highly recommend them. Eyeshades I’ve generally found less use for, and have traditionally only used them on planes. However, I recently spent a few nights sleeping in a camper van, and found them very useful in this setting a couple of times. Also, I’ve met lots of people who use them quite regularly and swear by them.


Van Der Werf YD, et al. Sleep benefits subsequent hippocampal functioning. Nature Neuroscience [epub 18 January 2009]

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