Variability in heart rhythm during sleep may help detect early signs of stroke, depression or cognitive dysfunction – even in people who appear to sleep normally, new research suggests.
The research centres on heart rate variability (HRV) – the natural variation in time between heartbeats – which reflects how the body’s automatic systems respond to stress, activity and rest.
HRV typically rises during the day as the body meets physical and emotional demands, then drops at night as the body shifts into recovery mode.
Researchers analysed data from 4,170 individuals over 13,217 person-years. They found that specific HRV patterns during sleep were linked to future diagnoses of various conditions.
Participants who later experienced stroke often had unusually high and erratic HRV.
Low HRV was common among those who went on to develop depression. People who later developed metabolic diseases showed high HRV with altered frequency patterns.
High HRV was also associated with increased risk of cardiovascular and endocrine diseases.
Lead author Dr Irina Filchenko said: “HRV matters for brain and overall health because it reflects how well the body regulates itself – primarily through the activity of the autonomic nervous system,” said
“This system controls vital unconscious processes such as breathing, digestion and muscle tone, helping the body maintain balance and adapt to internal and external demands.”
The study found that some participants had what appeared to be normal sleep based on standard measures such as sleep stages and minimal disturbance. H
owever, their HRV readings indicated hidden risks missed by conventional metrics.
Dr Filchenko said: “While many people are familiar with tracking sleep stages or total sleep time, nocturnal HRV provides a unique window into how the body functions during sleep.
“This is especially important because sleep is a critical time for many physiological processes underlying long-term health, such as cellular repair, memory consolidation, and the clearance of metabolic waste from the brain,” said
The researchers say HRV could act as an early physiological marker, revealing subtle dysfunction before symptoms or diagnoses emerge.
This could enable earlier intervention for diseases such as Alzheimer’s or stroke, where timing can affect outcomes.
Dr Filchenko added: “Some participants had ‘normal’ sleep according to traditional criteria, with little sleep fragmentation and the expected balance of sleep stages.
“However, HRV told a different story, picking up risks that the common sleep metrics missed. This suggests we need to rethink how we define and measure optimal sleep,”
The findings raise the possibility of using wearable devices to monitor HRV patterns over time.
Although current technology varies in accuracy, future improvements may support routine use in personal health tracking.
The researchers said: “The broader message is that sleep is not just a passive state of rest — it is an active, dynamic process that plays a vital role in maintaining long-term health, especially brain health.
“Our findings reinforce the idea that primary prevention matters, and that health problems start long before the clinical symptoms appear.”
