Is it true that older people get less deep sleep, and what does that mean?

Yes, this is true. Adults gradually lose deep sleep as they age. This is one of the most consistently demonstrated sleep changes associated with ageing, confirmed in a meta-analysis of 65 studies involving more than 3,500 participants. The steepest decline already occurs between your twenties and forties, not only at an advanced age. After the age of sixty, the loss appears to slow somewhat. In the Framingham cohort, people over sixty lost an average of 0.6 percentage points of deep sleep per year.

That decline is not harmless. The Framingham Heart Study, in which 346 people with an average age of 69 were followed for seventeen years, found that each additional percentage point of annual deep sleep loss was associated with a 27% higher risk of dementia. This is an observational study, so whether the loss itself is the cause has not been established. However, there is a biological reason to take it seriously: during deep sleep the brain clears harmful proteins involved in Alzheimer's disease. Sleep disruption promotes the accumulation of those proteins, and that accumulation in turn further disrupts sleep. This self-reinforcing pattern has been demonstrated in both animal research and in humans.

Deep sleep also affects your hormonal balance. Around seventy percent of growth hormone peaks in men coincide with the first period of deep sleep. As this sleep stage shortens, total 24-hour growth hormone production already drops two- to threefold between your thirties and forties. This likely explains a large part of the low growth hormone levels that accompany ageing.

Beyond deep sleep, other things change as well: you become sleepy earlier, wake up more often, sleep for shorter periods, and sleep efficiency declines. Researchers do temper this picture, however: in healthy older adults over sixty, many sleep parameters remain more stable than is often assumed. Caffeine is a point of attention, as older people are more sensitive to its sleep-disrupting effects than younger people.

Good sleep quality in younger and middle age is associated with better cognitive functioning later in life. In healthy older adults themselves, however, that association is less consistent: some studies find no effect or even a reversed one. What you can practically do with this: paying attention to sleep is most worthwhile before the age of sixty, and factors such as caffeine intake in the afternoon or evening and a regular sleep schedule are concrete starting points.