How does sleep help us process information?
There’s convincing evidence that memories are laid down during deep ‘slow-wave’ sleep. In your waking hours, when brain cells learn information, it goes into the hippocampus, the memory area of the brain. The memory is still quite fragile and, during sleep, neural networks are activated between the hippocampus and the rest of the brain.
Using EEG [electroencephalography], we see cycles of brain waves that are important for strengthening these memories. We’re looking into insight – deeper, qualitative information processing – which is more of an emerging field.
How did you test whether naps improve insight?
We developed a task using words associated with an emotion. We presented a word onto a screen for less than 50 milliseconds [one-twentieth of a second] then masked it, so nobody was actually consciously aware of seeing that word. We then presented another ‘target’ word that could be similar or dissimilar to the masked word: for example, participants might be shown the hidden word ‘bad’ then see ‘unhappy’ or ‘happy’, and we got them to press a button – labelled ‘good’ or ‘bad’ – and recorded how quickly they pressed. People were faster to respond if the word presented before was similar because dissimilar words take more time to process.
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Next, we gave the participants a period of wake or sleep, and did the same test. The people who stayed awake could watch films or read books, they just had to stay awake. The people who slept got to nap for 90 minutes.
The results showed that the people who napped were much quicker at responding to the target word. This is a fairly small study, with just 16 people and a wide range of ages. We need a bigger group and we’ll use EEG to identify the sleep stage which seems to predict performance on the task. We’ll also carry out the test overnight. Short periods of sleep might enhance aspects of memory and thinking, but if you have a 15-minute daytime nap, is that better than having 15 minutes extra of night-time sleep?
Why did you study naps?
We wanted to see whether sleep of any type would help people to process information that might enable them to make decisions. But in the day, we experience different hormone levels and light to the night, so this nap design controls those various factors. When we sleep, we go through stages of sleep, from light to deep and rapid-eye movement sleep – that full cycle lasts about 90 minutes. We got our participants to nap for that amount of time because that would capture most bits of sleep.
What are the practical applications?
We can look at people who don’t sleep well and see all sorts of problems, not just with cognitive and psychological health, but their general health as well. Some of my patients with mild cognitive impairment and dementia have problems with insight and decision-making, and we can see if there’s any scope for boosting that by modifying sleep. That can be through really simple things like sleep hygiene, but also more sophisticated brain stimulation using sound or drugs that can promote the deep sleep that might help with processing.
This is an extract from issue 329 of BBC Focus magazine.
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