Light at Night Linked to Thicker, Stiffer Heart Chambers in 11,000 Adults

Adults exposed to more light during the hours they were asleep had thicker and stiffer heart chambers, and early signs of reduced heart function, in a study of more than 11,000 people. The association held after accounting for the usual cardiovascular risk factors.

How it was measured

Light exposure was recorded by wrist-worn sensors rather than reported by the participants, which removes a large source of error -- people are poor judges of how dark their bedroom actually is. Cardiac structure and function were assessed by imaging. The analysis compared people across the range of night-time light exposure, from near-darkness to the levels typical of a room with a street lamp outside an uncurtained window.

What was found

Higher night-time light exposure tracked with greater left ventricular wall thickness and reduced compliance -- a heart that has become less elastic and does not fill as easily between beats. Those changes are not symptoms. They are the structural steps that precede diastolic dysfunction and, in some people, heart failure with preserved ejection fraction.

The mechanism, and its uncertainty

Light at night suppresses melatonin, shifts circadian timing, and raises sympathetic nervous system activity through the night when it should be falling. Any of those could plausibly load the heart over years. But the study observed people rather than assigning them to conditions, and the people who sleep in brighter rooms differ from those who do not -- they are more likely to be shift workers, to live on busy roads, to sleep less, and to be exposed to noise and air pollution at the same time. Those exposures travel together and are hard to separate.

What can be done about it cheaply

Of the modifiable cardiovascular risks, this is among the least expensive to address. Blackout curtains or a sleep mask, moving a charging phone out of the bedroom, and turning off standby lights cost little and carry no downside. None of that is established to protect the heart -- that is exactly what the study cannot show -- but the intervention is close to free and has a reasonable case behind it on sleep quality alone.

Bottom line

A dark bedroom is worth having regardless. Treat this as one more reason for it, not as a diagnosis, and keep the attention on blood pressure, lipids, smoking and activity, where the evidence is settled.