Most of us accept that getting enough sleep matters for our health. But a new study suggests that when you go to bed - and how much that varies from night to night - may matter just as much as the hours you clock up.
Why this is relevant in Britain
Cardiovascular disease is one of the most pressing health challenges Britain faces. Heart diseases cause around one in six deaths in the UK - approximately 110,000 people each year, and around 5 million people in the UK are currently living with heart disease. Any modifiable lifestyle factor that affects cardiac risk is therefore worth examining seriously.
Sleep is already one of those factors. Research from the Sleep Charity, published in 2024, found that 90% of 2,000 UK adults surveyed reported current sleep problems, with two thirds having experienced them for more than six years. A separate 2024 survey of 15,000 UK adults found that 60% of people reported sleeping six hours or less each night - significantly below the minimum threshold for healthy sleep. That's a substantial proportion of the population already carrying elevated risk.
What the new study found
A further study followed 3,231 people born in Northern Finland in 1966. Sleep patterns were recorded over one week at age 46 using activity monitors, and health outcomes were tracked for more than a decade using healthcare register data.
The researchers found that inconsistent bedtimes and greater variability in sleep timing were strongly associated with a higher risk of major cardiovascular events. The elevated risk was most pronounced in a specific combination: when measured time in bed also fell below eight hours, the risk was roughly twice thatof people with more regular sleep patterns.
One detail stands out as particularly worth noting: irregular wake-up times showed no clear link to cardiac events. It was variability in bedtime, specifically, that appeared to drive the association.
The caveats matter - read them
Before drawing firm conclusions, there are important limitations to be aware of.
This is an observational study. It identifies an association between irregular bedtimes and cardiac events, but it cannot establish that one causes the other.
People with highly variable sleep patterns may also have other risk factors - shift work, chronic stress, underlying health conditions - that independently affect cardiovascular risk. The study cannot fully separate these out.
The cohort was also drawn from a specific group: people born in 1966 in Northern Finland. Sleep patterns and cardiovascular risk profiles vary across populations, and the findings may not apply in exactly the same way to all demographics. Sleep duration and timing were estimated using activity monitors rather than clinical sleep studies, meaning the data captures time in bed rather than confirmed sleep duration.
That said, the study's strengths are meaningful: objective measurement rather than self-reported estimates, a large sample, and over a decade of follow-up.
Why sleep timing may matter for heart health
The biological mechanisms haven't been definitively established, but the hypothesis is plausible. Regular sleep timing helps maintain circadian rhythms - the body's internal clock - which governs processes including blood pressure regulation, hormone release, and inflammatory responses. Repeatedly disrupting these patterns may place additional strain on the cardiovascular system over time.
Practical steps worth considering
The study doesn't suggest you need to be in bed at precisely the same minute every evening. But it does reinforce what sleep researchers have argued for some time: consistency in when you sleep is part of what makes sleep restorative. That's a relatively low-effort, no-cost change - and one increasingly supported by the evidence.
