We are used to asking how many hours we slept at night, but a new study claims that this number is only part of the story. The time we spend in the various sleep stages, the number of awakenings, and the differences from night to night may also be linked to health in the coming years.
The researchers analyzed data from 95,559 participants in the British database UK Biobank, who wore a smartwatch on their wrist for seven consecutive days and nights. Afterwards, they tracked their medical records for a period of about 9 years and examined links between sleep characteristics and the emergence of 1,049 diseases and medical conditions.
The result was a vast map of statistical connections. More dream sleep and more deep sleep were linked to a lower risk for a series of diseases, while very short sleep, irregular sleep, and prolonged wakefulness after falling asleep were linked to a higher risk.
The bracelet tried to identify what happens during sleep
Participants were aged 56.2 on average and about 56 percent of them were women. The watch recorded movement for a week, and a machine-learning algorithm estimated total sleep duration, dream sleep, light sleep, deep sleep, wake time after falling asleep, and the degree of irregularity between nights.
This is an important advantage over questionnaires in which people are asked to remember how much they slept. On the other hand, an accelerometer does not measure brain waves, eye movements, breathing, or muscle activity as a laboratory sleep test does. The algorithm can estimate sleep stages according to movement, but it does not know with certainty when the person was in dream sleep or deep sleep.
Dream sleep stood out in particular.
Every addition of 47.6 minutes to dream sleep, known as REM, was linked to a lower risk for 83 diseases across 12 categories. The strongest connections were observed in neurological diseases, including Parkinson's, multiple sclerosis, dementia, and Alzheimer's, as well as in heart failure. For example, the hazard ratio for Parkinson's was 0.20 and for dementia 0.54.
It is important to note that these are impressive numbers, but they do not mean that an addition of dream sleep will prevent the diseases. It is possible that an undiagnosed disease is already altering sleep, especially in neurodegenerative brain diseases that can develop over years. The researchers attempted to reduce this possibility through an analysis in which they ignored diagnoses that appeared in the first two years, but only about 60 percent of the findings remained statistically significant.
Deep and regular sleep were also linked to health
More deep sleep was linked to a lower risk for seven conditions, including type 2 diabetes and major depression. Conversely, more light sleep presented a complex picture: It was linked to a higher risk for 28 diseases, but to a lower risk for other conditions, including heart failure.
Irregular sleep was linked to a higher risk for abdominal pain, anxiety disorders, and depression. Prolonged wakefulness after falling asleep was linked to six conditions, including psychoactive substance dependence and harmful alcohol use. Here, too, the direction is unclear. Anxiety and depression can disrupt sleep, and fragmented sleep can, for its part, worsen a mental state.
How many hours should you sleep?
When the researchers examined total sleep duration, the lowest risk for most conditions was found in the range of six to eight hours a night. This range was linked to benefit in 69 conditions. Sleeping less than five hours was linked to a higher risk in 37 conditions, including dementia, heart failure, and type 2 diabetes.
The finding does not mean that six hours is an appropriate target for everyone or that eight hours is a limit that must not be exceeded. Sleep needs vary between people and across the lifespan. Additionally, long sleep can be a sign of illness, cumulative fatigue, or medication, and not necessarily the cause of the problem. The reasonable message is that consistently very short sleep is not a condition to accept.
The results reinforce the understanding that good sleep is not just counting hours. A regular routine, sleep continuity, and feeling refreshed in the morning are no less important. Anyone who regularly sleeps less than five hours, wakes up frequently, snores heavily, experiences breathing pauses, or suffers from severe daytime fatigue should seek medical evaluation.
Smartwatches or smart pulse bands can be a useful tool for identifying ongoing changes, but the body remains the more important indicator. If the watch gives a low score and you feel great, there is no need to panic. If it displays good data but you are exhausted, listen to yourself rather than the screen.