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.