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Dark Matters: Effects of Light at Night on Metabolism

Hayley Dawes
2 September 2022

The term ‘sleep hygiene’ encompasses the habits and environmental factors that determine whether you enjoy a glorious night’s rest or find yourself glumly scrolling through Instagram at 4am. Are you exposed to even small amounts of light throughout the night? If so, this constant low level of light may be having negative effects on your metabolism. 

We all know caffeine and alcohol play their part, and perhaps you’ve learned not to binge-watch Netflix in bed. But there’s another crucial factor you might be overlooking – and it’s harming more than just your shut-eye.

By now, research has made it clear how important sleep is for good health and longevity. Following a normal sleep-wake cycle translates into better brain performance and a lower risk of certain diseases. But did you know that your light-and-dark cycle can have a serious impact on your health as well? Studies show that following a natural light-and-dark cycle (bright light during the day and no light at night) may improve your body’s metabolism, helping your body better regulate itself, while too much artificial light could harm it. Be mindful of how much artificial light is in your home at night.


The Connection Between Metabolism and Light

Your metabolism – the process by which your body turns nutrients into energy – doesn’t stay the same throughout the day. That’s why walking at different times of day burns a different number of calories. Adults also use more energy in the afternoon and early evening.

According to scientific research, the metabolic effects of dim light at night may have very real health consequences. Oscillating cells in our brain maintain our circadian rhythm. This activity is lessened by constant exposure to light. The result is that many metabolic processes in our cells are uncoordinated and do not run as efficiently. Over time this lower metabolic rate leads to physical symptoms such as weight gain and fatigue.

Exposure to dim light at night appears to amplify the effects of bad choices, causing us to gain more weight and suffer more inflammation than we would if we slept in total dark. Night-time light also appears to stimulate eating at odd hours, leading to a higher overall caloric intake and the resulting higher body mass.


Light and Your Circadian Rhythm

Although our bodies have internal clocks that regulate our circadian rhythm even in the absence of external cues, we also set our clocks partly by cues from the outside world. Light is a major circadian cue. When our retinas sense light, the message is passed to the suprachiasmatic nucleus of the hypothalamus of our brains. The suprachiasmatic nucleus in turn tells the pineal gland not to make melatonin.

Melatonin is a hormone that supports sleep and the many cell activities that occur during sleep. When we are exposed to even small amounts of light, we make less melatonin. This is fine during the day when we need to be awake. However, it can be detrimental at night when our bodies need melatonin to help us get the high-quality sleep we need. With years of exposure to light at night, several cell processes including metabolism may be affected.

Maintaining a healthy circadian rhythm is part of a healthy lifestyle. If you find that you are having trouble sleeping or trouble keeping the pounds off, limiting night-time light exposure may be an effective first step to sleeping better and revving up your metabolism. Light-blocking curtains, and other measures that correct the circadian rhythm may help you to attain the active metabolism and energy that you need to be your fittest, healthiest self.


The Problem of Light Pollution

Light pollution is a very real health problem for people who live in areas where there are nearby sources of light such as streetlamps and lighted parking lots. This light is a nuisance, keeping us awake and preventing us from seeing the stars.

We now have more devices than ever that emit various types of light. Our smartphones and laptops stream out blue light, while our indoor lamps and overhead lights flood our spaces with white light. There is increasing evidence that indoor light exposure, particularly at night, can contribute to circadian rhythm disorders that have long term health impacts.


Light Exposure and Sleep

Light is the most important external factor affecting sleep. While most people intuitively know that it’s easier to sleep when it’s dark, the link between light and sleep goes much deeper.

Recent research has found that even moderate light exposure during sleep – the flash of a passing car or the glimmer of an LED alarm clock – can prevent your body from properly winding down. This has a negative impact on your heart health and blood sugar levels, affecting your risk of diabetes, heart disease, and even obesity. Researchers found evidence that sleeping in a moderately lit room increases your heart rate – which, even if you are happily snoring, activates your sympathetic nervous system. At night, your parasympathetic should be running the show, allowing for proper recovery, but light exposure jeopardises this process.

Furthermore, people who slept in a brighter room were more likely to show signs of insulin resistance the next morning, meaning their bodies struggled to use glucose from the bloodstream for energy, leading to a rise in blood sugar. This adds credence to research suggesting a link between poor sleep (even in those who aren’t fully aware of it) and weight gain.

Fortunately, there are simple solutions: invest in black-out blinds and keep your devices out of sight, or pop on an eye mask. Rest assured; it makes a difference.


What Affects Our Blood Sugar?

Night-time light isn’t the only factor that can mess with your blood sugar… 

Poor hydration

This increases levels of the hormone vasopressin, which studies have linked to a rise in blood sugar. Another reason to keep a chilled glass on your desk.


Strange as it might sound, sunburn can cause a spike in glucose levels – your body’s response to stress. Don’t forget to apply your SPF.

Gum disease

This creates inflammation, which throws your body’s defence system out of balance and impacts glucose control. Don’t skip the dentist.