So if we imagine for a moment that older people do need the same amount of sleep, why then do they sleep for fewer hours? One hypothesis is that the aging process disrupts their circadian rhythms, causing them to wake earlier than they should. Studies have demonstrated the clock does seem to shift, leading people to wake earlier in the morning and go to bed earlier at night. They might still need the sleep, but they can’t get it and when they do fall into a slumber, the quality of sleep is not as good as when they were younger.

In a new study from Russia, 130 people went to a laboratory one morning and then stayed there all day and overnight. Staff kept them awake for the entire time, regularly asking them to assess how sleepy they felt. These feelings of sleepiness vary throughout the day and night and in sleep deprivation experiments such as this, they are taken to reflect processes related to the body clock such as changes in body temperature at different times of day and the release of the hormone melatonin in the evening.

The slow-wave activity in the volunteers’ brains was also measured several times during the day and night. Then all this data was analysed in relation to a sleep diary the people had kept for the previous week, in order to see how the pattern of sleepiness and slow brainwaves varied according to propensity to be morning or evening types. They found that, once again, the older people felt sleepy at different times from the younger people and had different timings of slow-wave activity in the brain.