An invisible clock ticks incessantly in your body. That 24-hour cycle, or your circadian rhythm, determines when you sleep, wake up, eat and recover. As long as that internal timing is tight, hormones, organs and your immune system work together in remarkable harmony. But when the clock becomes disrupted, the damage appears to extend far beyond a bad night’s sleep.
This is evident from a large study from 2025 among more than 2,000 elderly people with an average age of 79 years. The outcome: people with a strong and regular day-night rhythm were almost half as likely to develop dementia as their peers with a messy biological clock. Over a three-year period, 10 percent of people with a disturbed rhythm developed dementia, compared to 7 percent of those with a more stable rhythm.
This is not just about sleep duration. The circadian rhythm also regulates heart rate, body temperature and hormone release. The study took into account factors such as high blood pressure and heart problems, which are often associated with poor sleep. Surprisingly enough, sleep apnea, in which breathing continues to stop during the night, was not included. And there is a complication: sleep apnea is more common in people who are overweight, have diabetes and have an unhealthy lifestyle.
Chicken or the egg
Researchers have been wrestling with the chicken-or-the-egg question for years. Poor sleep has been suspected as a risk factor for both heart disease and dementia. At the same time, incipient brain damage can disrupt the sleep-wake rhythm. The recent update from the dementia committee of The Lancet is therefore cautious: sleeping too long or too short does not appear to be an independent cause of dementia. Night work does increase the risk, but surprisingly not consistently more than day shifts. If sleep alone was the culprit, there should be that difference.
That points to something else: disruption of rhythm and routine. Shift work is associated with stress, poor nutrition, smoking, alcohol, less exercise and social isolation. These are all factors that in themselves are linked to brain aging. The problem is therefore broader than ‘just’ too little sleep.
There is also the popular theory that sleep helps clear toxic proteins in the brain, such as amyloid plaques that are characteristic of Alzheimer’s. During deep sleep the brain would undergo a kind of cleaning. Makes sense, but the evidence is mixed. Some animal studies even show the opposite. And mice are simply not people, especially when it comes to sleep later in life.
Exercise more
The connection with sleep is therefore unclear. What does work is surprisingly old-fashioned: exercise every day. Half an hour of moderate exercise, preferably outside, strengthens the circadian rhythm, improves sleep and at the same time protects against heart disease and dementia.
So no, you don’t have to ‘optimize’ your sleep with pills and gadgets. But your biological clock does require regularity, daylight and exercise. Or to put it more simply: put on a jacket and go for a walk. Your brain will thank you.