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Scientists comment on an observational study in European Heart Journal on light exposure at night and changes to heart structure and function.
Dr Darren Rhodes, Lecturer in Cognitive Psychology, Keele University, said:
“This is a strong observational study. The sample is large, light exposure was measured objectively using wearable sensors, and the authors used detailed cardiac MRI alongside extensive statistical adjustment and sensitivity analyses. The association between greater nighttime light exposure and differences in cardiac structure and function therefore looks credible.
“However, the causal wording in the press release is too strong. This study shows an association, but it cannot demonstrate that nighttime light caused the cardiac changes. The reported differences in cardiac measures are also relatively small, mostly around 1 to 3%, so claims that nighttime light is directly ‘damaging’ individual hearts would go beyond the evidence.
“From a light and circadian science perspective, an important limitation is that the sensor measured light at the wrist for only seven days. It also measured illuminance rather than the spectral composition of the light, and the sensor was worn on the wrist rather than at eye level. So this is very useful real-world evidence, but it is not a precise measure of the biological light dose experienced by each participant.
“Overall, this adds good evidence that nighttime light exposure may be relevant to cardiovascular health, but intervention studies are still needed before we can conclude that reducing nighttime light exposure will reduce cardiovascular risk.”
Dr Sonya Babu-Narayan, Clinical Director at the British Heart Foundation, said:
“Exposure to light at night can disrupt the body’s internal clock and affect the duration and quality of sleep, but this is an observational study, so it cannot prove that nighttime light directly caused the subtle changes in the structure and function of the heart. The links seen do not imply causation nor imply the findings are preventable.
“The researchers accounted for many factors that can influence heart health, however, it’s still possible that other unmeasured factors contributed to the results. People who had more night-time exposure to light when they filled in their 7 day diary might differ from others in ways that affect their future heart health. The findings also suggest that not sleeping for long enough could explain the link to higher risk of cardiovascular problems. More research would be needed to understand whether reducing nighttime light can directly improve cardiovascular health, what mechanisms underly the association and how important they are together with co-exposure to other factors and how this might affect groups likenight time shift workers.
“We do know that getting enough good quality sleep, on average 7 hours a night where possible, and avoiding blue light at night is good for your health. Meanwhile, if you are someone who needs to have a light on at night the individual risks are low, the heart changes were subtle and other factors may be more important for you to address so there is no need to be anxious or lose more sleep over it.”
Prof Jill Belch, Professor of Vascular Medicine, University of Dundee, said:
“It is already recognised that light at night adversely affects animals and insects, thus human harm is not unexpected. It is an interesting and well-conducted study which adds to growing evidence that exposure to light at night is associated with poorer cardiovascular health. A particular strength is that the researchers actually measured individuals’ light exposure and then used detailed MRI scans to look at the structure and function of the heart.”
Dr Jeffrey Kelu, Research Associate, King’s College London
“Dai et al. (2026) provide important new evidence linking greater nighttime light exposure with adverse changes in the heart. They showed that higher exposure was associated with less favourable cardiac structure and subtle declines in function on cardiovascular magnetic resonance (CMR), with a clear dose-response relationship.
“These findings are potentially clinically relevant because greater nighttime light exposure was also associated with higher subsequent risks of several cardiovascular disease outcomes and cardiovascular mortality. Previous studies have linked light at night with increased cardiovascular risk, but this study adds detailed imaging evidence of early, subclinical cardiac changes that may precede overt disease.
“The authors found that people with greater nighttime light exposure also tended to sleep for less time, and this shorter sleep appeared to explain part of the link with adverse changes in cardiac changes. This is biologically plausible, as light exposure at night can suppress nocturnal melatonin release and disrupt both sleep and circadian timing.
“A major strength is the ability to link detailed CMR phenotyping in around 10,000 participants, a scale that is typically difficult to achieve, with individual wearable light measurements. This is further improved by measuring participants’ actual light exposure, rather than relying on satellite estimates of outdoor nighttime light as many previous studies have done.
“There are, however, several important limitations. Light exposure was measured for only one week, whereas the CMR was performed approximately three years later. The analysis therefore assumes that the week of wearable measurement reasonably reflects participants’ longer-term habitual nighttime light exposure. As an observational study, it also cannot establish causality.
“Residual confounding is also possible. For example, nighttime light exposure may occur alongside other environmental factors such as noise, which could independently affect sleep and cardiovascular health. The wearable sensor measured light intensity but did not capture its spectral composition. This is relevant because the circadian system is particularly sensitive to blue light.
“The UK Biobank population is also predominantly White, generally healthier than the wider UK population, and the participants in this analysis were mostly older adults. This limits how confidently the results can be generalised to younger and more diverse populations.
“Finally, night-shift workers were excluded. This is an important limitation because they are among those most chronically exposed to light at biologically inappropriate times and already have an increased risk of cardiometabolic disease. Whether the same cardiac changes are seen, or are even stronger, in shift workers remains an important question.
Dr Karl Lawrence, Lecturer in Drug Development, King’s College London, said:
“This is an interesting study that builds upon increasing evidence on light exposure and cardiac impact (both beneficial and detrimental). The methodology used to determine night time light exposure is an improvement on previous studies. However, there are still significant limitations in the light measurement that could have a significant impact on the findings.”
“These limitations are declared but not to the extent they may impact the study. As an epidemiological study this can only demonstrate correlation and not causation, and so further studies would be needed to demonstrate causality here.
“The biggest weaknesses of the study (some of which are declared) from a photobiology point of view are:
The light sensor doesn’t measure the spectrum of light – the effect of this is understated as spectrum determines the melatonin disrupting dose. E.g. is the light coming from a red LED on an alarm clock or white light from a street light? The two could give a similar reading but have very different effects on melatonin.
The use of a light sensor on the wrist isn’t a good proxy for the dose received by the eye – this could be both over or under estimation.
The skin of the eyelid would preferentially filter more of the melatonin disrupting blue light than other less consequential wavelengths, so the wrist dose and spectrum isn’t the same as that received by the eye.
The cohort is ~98% White, so the findings can’t be assumed to generalise to other ethnic groups, particularly as darker skin is known to attenuate light more than white skin.
The high dose group is skewed towards summer months, suggesting the increased light may be from earlier sun rises. This has behavioural impacts – for example, participants may mitigate this with the use of eye masks to prevent early waking, so any dose the eye received could be negligible.
Although acknowledged and some supporting evidence is given, one week doesn’t equal general exposure. Again, as the high group was skewed towards summer months this suggests seasonality, so the same person in winter may be in the low group. Then, to correlate this to heart scans three years later feels like it is somewhat of a stretch.
“These weaknesses in the study feel to me like the findings are overstated. This is also true in the editorial, particularly the claim “The takeaway is clear, darkness deserves recognition as a vital sign, as essential to cardiovascular health as blood pressure control and clean air” – this goes beyond what the evidence supports. Again, this study can at most suggest a correlation between darkness and cardiovascular health. This is a good evidence base to justify further investigation, but more evidence is needed before we can make strong conclusions about the impact of light on cardiovascular health.”
‘Nighttime light exposure and cardiac structure and function’ by first author et al. was published in European Heart Journal at 23:05 UK time on Wednesday 9 September.
DOI: https://doi.org/10.1093/eurheartj/ehag563
Declared interests
Dr Darren Rhodes: Nothing to declare
Prof Jill Belch: No COI
Dr Jeffrey Kelu: “I have no conflicts of interest to declare, aside from a small research grant awarded by PCR Biosystems Ltd in 2024 (£1,000).”
Dr Karl Lawrence: “I previously received funding from the sunscreen industry and I have two patents related to this work. I have previously worked for a QMUL spin out company called Keratify and I am currently founder for a KCL spinout company called Epinostics.”
For all other experts, no reply to our request for DOIs was received.