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Study Finds That Sleeping With the Light On is Bad For Your Health


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Medically reviewed by Kathy Shattler, MS, RDN

CNN is one of many news sites that are now reporting on a new study that revealed that moderate light exposure during sleep harms heart health and increases insulin resistance. While it’s commonly known that having the light on while in bed keeps the brain awake, now it’s known that even the smallest amount of light may affect young people’s health. 

The new Northwestern Medicine study authored by Dr. Phyllis Zee, Chief of Sleep Medicine at Northwestern University Feinberg School of Medicine and a Northwestern Medicine physician, showed that artificial light also raises the risk of heart disease and diabetes. The study was published Monday in the Journal of the Proceedings of the National Academy of Sciences.

It is stated in the Journal that “The primary finding of this study is that exposure to a single night of room light (100 lx) during sleep increased measures of insulin resistance the next morning. Interestingly, the effect of nighttime light exposure on metabolic function was correlated with an increase in sympathovagal balance during sleep.” The sympathovagal balance is the main determinant of blood pressure variability.

From Moderate Ambient Lighting and Dim Lighting to Bright Light  and TV Set Lighting

Daily Science states that “A significant proportion of individuals (up to 40%) sleep with a bedside lamp on or with a light on in the bedroom and/or keep the television on.” This is a large portion of individuals who are exposing themselves to health risks.

When it comes to light level, dim overhead lights or even street lights coming in through a window all pass through the human eyelid. According to the news source, it is estimated that about five to ten percent of environmental light gets through the closed lid to the eye. 

“Yet even that tiny amount of light created a deficit of slow-wave and rapid eye movement sleep, [and] the stages of slumber in which most cellular renewal occurs.”

What Happens When You Sleep with Light on?

According to a study, sleeping with even a small amount of light may harm your health in many ways, including the following:

  • Raised blood sugar levels flood the body with extra insulin to overcompensate until the body eventually loses its ability to do so. This may cause insulin resistance over time and ultimately lead to Type 2 diabetes
  • Raised heart rate, which is a risk factor for future heart disease and early death
  • An imbalance in the sympathetic (fight or flight) and parasympathetic (rest and relax) nervous systems, which has been linked to higher blood pressure 

Daily Science explains further, “… Insulin resistance occurred the morning after people slept in a lighted room. Insulin resistance is when cells in your muscles, fat, and liver don’t respond well to insulin and can’t use glucose from your blood for energy. To make up for it, your pancreas makes more insulin. Over time, your blood sugar goes up.” This is called a state of insulin resistance.

How Bright is too Bright at Night?

Harvard Health Publishing points out that “Even dim light can interfere with a person’s circadian rhythm and melatonin secretion. A mere eight lux—a level of brightness exceeded by most table lamps and about twice that of a night light—has an effect.”

Recommendations to Keep the Light Out at Night

“Light and its relationship to health are double-edged. In addition to sleep, nutrition, and exercise, light exposure during the daytime is an important factor for health, but during the night, we show that even modest intensity of light can impair measures of heart and endocrine health,” added a news source

With this, it is recommended to let the light shine in during the day but keep it out during the night. The study recommends the following to darken your room at night: 

  • Close your blinds and curtains 
  • Use room-darkening blinds
  • Remove all technological devices from your room before going to bed
  • Turn off all the lights
  • Use a sleep mask.
  • Pay attention to the light in your bedroom, and make sure that you start dimming your lights at least an hour or two before you go to bed to prepare your environment for sleep.
  • Remove sources of light in the bedroom that are not necessary
  • Should a night light be needed, keep it dim and at floor level
  • Be aware of the type of light in your bedroom and avoid using lights in the blue spectrum and those emitted by electronic devices like TVs, Smartphones, and laptops. Change the light color if needed to reddish and brownish tones and opt for LED lights.

Harvard Health Publishing adds further recommendations for individuals to protect themselves from blue light at night:

  • Use dim red lights for night lights. Red light is less likely to shift circadian rhythm and suppress melatonin.
  • Avoid looking at bright screens beginning two to three hours before bed.
  • If you work a night shift or use a lot of electronic devices at night, consider wearing blue-blocking glasses or installing an app that filters the blue/green wavelength at night.
  • Expose yourself to lots of bright light during the day, which will boost your ability to sleep at night and your mood and alertness during daylight.

Medically reviewed by:

Kathy Shattler

Stacey Rowan Woensdregt has more than 15 years of experience in print media, online media, copywriting, and digital marketing. She has written for many bespoke magazines and media houses and has worked within top digital marketing agencies around the world. Her niche markets include architecture, property, health and wellness, holistic medicine, art and lifestyle, and business.

Medically reviewed by:

Kathy Shattler

Harvard Health Publishing

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