Jan 15, 2026
3 min read
Inside the body, a 24-hour rhythm known as the circadian rhythm quietly coordinates when we sleep, wake, eat and recover.
Inside the body, the number of 24 hours is known as the circadian rhythm.This internal clock system helps keep organs and hormones in sync.
When it's disrupted, the effects can go beyond poor sleep and have long-term impacts on brain health, according to growing evidence.
A large study of more than 2,000 people with an average age of 79 in 2025 found that those with strong circadian rhythms had nearly half the risk of developing dementia.Circadian rhythms regulate daily processes including sleep timing, hormone secretion, heart rate, and body temperature.
Related: Dementia Can Steal Your Cell Clock, Suggest New Treatments
During the three years of follow-up, 7 percent of participants with irregular body clocks, measured using heart rate monitors, developed dementia, compared with those with regular rhythms.
Disrupted circadian rhythms are often associated with poor sleep.For decades, poor sleep has been suspected of contributing to both depression and heart disease, which share multiple risk factors.
In the 2025 study, poor heart health and high blood pressure, commonly associated with sleep disturbances, were taken into account in the analysis.However, it was not sleep apnea.
Sleep apnea is a common condition in which breathing stops and starts repeatedly during sleep, which reduces oxygen supply to the brain and raises blood pressure.Its link with dementia remains controversial, especially because sleep apnea is more common in people who already have dementia risk factors, such as obesity, diabetes, smoking, and alcohol abuse.
This overlap makes it difficult to determine whether apnea itself increases dementia risk or reflects broader metabolic and cardiovascular vulnerability.
The review suggested that addressing physical inactivity associated with disrupted sleep fatigue is a promising approach.Simracing activity reduces obesity, improves sleepy-type dementia, and supports multiple pathways simultaneously.
Other explanations may also help clarify the link between disrupted circadian rhythms and dementia.One involves the immune system, which is affected by circadian rhythms as well as sleep and plays a role in both heart disease and neurodegeneration.
Another theory is that sleep helps remove toxic proteins from the brain, including the amyloid plaques that are a hallmark of Alzheimer's disease, through waste disposal systems that appear to become more active during sleep.
This permissive hypothesis is widely disputed, but the evidence remains mixed.Some animal studies, including rats, show that the release of toxins during sleep decreases rather than increases.Mice sleep patterns differ significantly from humans, especially in later periods, so animal findings should be interpreted with caution.
The Lancet Commission update on the prevention of dementia concluded that long or short sleep may not be a true independent risk factor for dementia.Another reason for this conclusion is that evidence from worldwide sleep disorders does not clearly point to sleep time as a major driver of risk.
Although shift work has been associated with an increased risk of dementia, studies have not shown a higher risk for night shift workers compared to day shift workers.If only sleep disturbances were responsible, night shift workers would be expected to have a higher risk.
This suggests that circadian disruption may be important regardless of sleep time.However, shift work is associated with unhealthy lifestyle habits, including poor diet, smoking, alcohol consumption and inconsistent exercise.
Chronic stress, lack of routine, hormonal disturbances, high blood pressure, reduced opportunities for recreation, and social isolation also cluster in shift work.Each of these factors is independently associated with dementia, heart disease, and sleep deprivation, only complicating isolating the effects of circadian disruption.
The Lancet authors said that if amyloid release occurs in humans, it usually occurs in the first two hours of sleep, when deep sleep is most common.Deep sleep is not preserved even if the total sleep is less than seven hours.
Therefore, they suggested that fragmented sleep and disrupted biological rhythms may be an early consequence of dementia-related brain changes rather than the cause.Toxic plaque can build up in areas of the brain that control sleep and wakefulness long before memory problems develop.
So should sleep deprivation be used to prevent dementia?The Lancet's advice was not to restrict sleep.
The evidence linking long sleep, defined as more than eight hours, to dementia risk is not supported when more data is considered.
A recent study tested an individualized program combining several methods, such as light exposure, sleep planning, daytime activity, and caregiver support, to improve sleep in people with dementia.After eight months, sleep improved in the intervention group, although sleep improved with general concern.
Overall effects were small to moderate and did not improve depression-related behaviors or health outcomes.Although these methods may benefit caregivers by providing routine, their impact on people with dementia appears to be limited.
Both acute and chronic sleep deprivation, especially loss of deep and REM sleep, can impair memory.It remains uncertain whether long-term disruption of restorative sleep increases the risk of dementia later in life and whether treating sleep problems can prevent dementia.
Because improving sleep is viewed as a potential preventive strategy, medications used to treat insomnia are also worth investigating.Sedative medications such as benzodiazepines have been associated with an increased risk of dementia, as have daytime sleepiness, falls, and accidents.Melatonin, although used by many people, has not shown consistent benefits for improving sleep in adults.
That said, there are evidence-based ways to improve sleep and support healthy circadian rhythms.
Regular moderate exercise, especially outside and before noon, for about 30 minutes a day is one of the most effective forms of exercise.As a bonus, exercise is one of the most protective factors against heart disease and dementia.
So get warm and start walking.
Eef Hogervorst, Professor of Biological Psychology at Loughborough University
This article is reprinted from The Conversation under a Creative Commons license.Read the original article.
