MADJAR

Danny tells me he can’t sleep at night. “Ivan is nocturnal,” he says, “it’s normal for a cat.” They rest during the day and spring to life after dark. Ivan wanders the house looking for company–someone to play with, someone to curl up next to, someone to entertain him. Loneliness? Boredom? Who knows? The mind of a cat is a mystery.

When Danny finally drifts off, Ivan leaps onto his head. The cat, apparently, has a fondness for well-styled hair. Toes, however, he despises. Even when they’re buried under a blanket, Ivan sees feet as sworn enemies and pounces on them with wild conviction. Naturally, Danny wakes in panic.

Half asleep, he carries Ivan out of the bedroom and closes the door. In protest, the cat meows for hours and scratches the door without pause. He’s lonely, Danny tells me. It just wants company.

Danny’s nightly saga reminded me of a recent article I read in JAMA Network Open, a study that sheds scientific light–quite literally–on what happens when our nights are not dark enough. The researchers investigated whether exposure to light at night, even low levels, might raise the risk of cardiovascular disease.

The study enrolled 88,905 adults aged 40 and older from the UK Biobank. Unlike studies that rely on self-reported “I think my room is dim,” this one used objective measurements. Each participant wore a wrist-mounted light sensor for one full week. These devices measured light intensity minute by minute, capturing both daytime brightness and nighttime illumination. The result was an astounding 13 million hours of light-exposure data–one of the largest datasets of its kind ever analyzed.

Then came the long wait. Over 9.5 years, the researchers tracked who developed coronary artery disease, myocardial infarction (heart attack), heart failure, atrial fibrillation, or stroke. They used the UK National Health Service database — meaning diagnoses were based on medical records, not memory or self-report.

Crucially, the researchers performed a series of increasingly rigorous statistical adjustments to control for just about every major cardiovascular risk factor: age, sex, BMI, smoking status, alcohol consumption, physical activity, diet quality, medication use, sleep duration, socioeconomic deprivation, and even polygenic risk scores–a measure of inherited susceptibility to heart disease. In other words, they tried very hard to make sure that the “light at night” effect wasn’t simply masking some other cause.

And still, the findings were striking. Compared with those who slept in the darkest environments, participants in the brightest-night group had significantly higher risks: 32% higher risk of coronary artery disease; 47% higher risk of myocardial infarction; 56% higher risk of heart failure; 32% higher risk of atrial fibrillation; and 28% higher risk of stroke.

The study joins a growing body of research linking circadian disruption–caused by nighttime light, shift work, erratic sleep patterns, and even the glowing screens we cradle in bed or just before falling asleep–to higher risks of hypertension, obesity, diabetes, and cardiovascular disease. Previous laboratory studies have shown that even dim light (as low as 5-10 lux) during sleep can increase sympathetic nervous system activity, impair glucose metabolism, and raise nighttime heart rate. Observational studies from Japan, Korea, and the U.S. have found similar associations between nighttime brightness and metabolic and cardiovascular risks. In short, the JAMA study does not stand alone; it strengthens a signal that scientists have been detecting for years.

After finishing the article, I called Danny immediately and told him about the study. “You need dark, quiet nights,” I said.

We brainstormed enthusiastically over the dinner table, searching for the best strategies for Ivan the Terrible: a few minutes of vigorous evening play to burn off pent-up energy, a satisfying late meal, a designated nighttime territory–a warm cat bed, a perch near a window, even a cozy blanket in a quiet corner outside the bedroom — and a closed bedroom door to consistently reinforce the message that nighttime is for sleeping, not for acrobatics.

The broader message is one we all — not just cat owners — should bear in mind: quality sleep is essential. It is not just about feeling sharper the next day. Good sleep regulates our metabolism, lowers inflammation, stabilizes blood pressure, and protects the delicate rhythms that govern heart health. Disrupt those rhythms often enough — whether by light, noise, stress, or a restless cat — and the consequences may reverberate through the body for years.