Human beings were made to be awake and active during the daytime, and to sleep and rest at night. That cycle becomes the biological norm from which we function and remain healthy. The system is controlled by “light sensitive neurons”, or nerve cells, in a specific area in the deeper parts of the brain. This area has specific functions that respond to environmental stimuli. One such stimulus is sunlight, or light in general.
The cycle referred to above is called the Circadian Rhythm. We are “wired” in such a way that our bodies have a 24-hour schedule which is “synchronized” with the 24-hour schedule of a day. We’re awake during the daytime and asleep at night. The Circadian Rhythm is the reason children have a mild fever during the daytime, but have a temperature of 103° at 10:00 pm, or why grandpa gets more confused and belligerent when the sun goes down, and you’re ready to go to bed.
Disruptions of the Circadian rhythm are common, and can be problematic. Flying from the U.S. to Europe or Asia, across many time zones, and developing jet lag is one such situation. Working the night shift, when everyone else is sleeping, is another. The natural Circadian cycle is disrupted, temporarily throwing you into a physically bothersome state. You eventually adapt to the change, but until you do, life is difficult.
To see if Circadian disruptions had any long term adverse effects on people, researchers in Boston studied the effect of sleeping at night in rooms with various degrees of light. There were 90,000 patients who were subjected to sleeping in dark, mildly lit, moderately lit, or brightly lit rooms. I’m uncertain how they did this, but after 8-10 years they were evaluated for adverse changes in their lives. People who slept in the brightest rooms had a 30% to 50% higher risk of cardiovascular disease than those who slept in the darkness. These diseases were heart failure, heart attacks, coronary artery disease, atrial fibrillation, and stroke—serious outcomes.
Compared to patients who slept in near total, or total, darkness, the following results were obtained by those who “slept” with the brightest light on:
56% had a greater chance of heart failure
47% were more likely to have a heart attack
30% were more likely to develop coronary artery disease, A Fib, or stroke
There was no down side to sleeping in a dark room. Because of natural aging, those folks had problems, too, but far less frequently than those who slept in a lighted room. They also found a dose-response relationship: the brighter the light, the more likely people were to experience cardiovascular disease outcomes.
A link between Circadian rhythm and cardiovascular function is well established. The American Heart Association, the AHA, says any disruption of the Circadian rhythm increases the risk of hypertension, disrupts regulation of the blood pressure, and increases cardiovascular disease risk. When nighttime light exposure disrupts Circadian rhythm, over time, the normal functional systems of our bodies are affected and peoples’ risk of cardiovascular disease worsens.
How much light is too much? If you must have light in your bedroom, it should be no brighter that the equivalent of 5 candles at arm’s length away. That sounds like a lot of light, but “they” say that amount will not affect the Circadian rhythm. If the light has a blue tint, it has greater cardiovascular disease effect. The bluer the light the greater the adverse effect. Also, a night light you plug into the wall outlet has little or no risk.
If you’re a patient with insomnia who is at high risk for cardiovascular disease, and you sleep with the light on, you should turn it off. Make your bedroom as dark as possible, using any means you have available. There is “no downside to darkness” when it comes to adverse cardiovascular outcomes. Get over your fear of the dark and realize light in your room may discourage human intruders, but it disrupts your normal biological rhythms and emboldens cardiovascular disease.
Reference: Koenig D. Sleeping With the Light on May Increase Heart Disease Risk Medscape 2026 January 14.
