Light Exposure and Body Weight: Is There a Link?

Light Exposure and Body Weight: Is There a Link?Morning sunlight – it wakes you up and helps you shake off that half-asleep feeling, but can light exposure help you control your body weight too? That’s what a new study published in the journal PLOS ONE would seem to suggest.

Early Morning Sunlight – Can It Help You Stay Lean?

Researchers at Northwestern University’s Feinberg School of Medicine asked 54 healthy volunteers to wear a wrist monitor for seven days. This special wrist monitor recorded how much light they were exposed to and its intensity at different times of the day. They also recorded everything they ate and drank over the course of the seven-day study.

After considering all the variables, the researchers found a strong link between the participants’ BMI and the amount, timing, length and intensity of their exposure to natural light. Subjects exposed to bright light in the morning had a lower BMI relative to those that got more of their light exposure later in the day. Apparently, the earlier the better. Each hour delay in soaking up bright light in the morning was linked with an increase in BMI.

Even after controlling for other factors that influence BMI like age, gender, physical activity, sleep timing and season, the link between early morning bright light exposure and BMI persisted. Makes you want to throw open the curtains and soak up some light when you wake up, doesn’t it?

BMI and Light Exposure: The Importance of Circadian Rhythms

Although this was a small study, there’s growing evidence that circadian rhythms, biological changes that follow a 24-hour cycle affect health in a variety of ways. Circadian rhythms are governed by our internal biological clock, a set of nerve cells that lie in a portion of the brain called the suprachiasmatic nucleus located in the hypothalamus. Our biological clocks are influenced by input from the outside, especially light exposure. Changes in light exposure have the power to disrupt the activity of our biological clock and throw off circadian rhythms.

What happens when circadian rhythms are disrupted? It throws off your sleep-wake cycle, leading to insomnia or daytime sleepiness, alters the release of hormones, causes mood changes and changes in body temperature. If you’ve ever taken a trip overseas to a different time zone and felt irritable and groggy due to jet lag, you’ve felt the effects of circadian rhythm disruption. It probably took several days for you to adjust. People who work third shift and sleep in the day also experience a disruption in their natural rhythms. In fact, working third shift has been linked with an increased risk for health problems like breast cancer and heart disease.

Interestingly, abnormal circadian rhythms have been linked with type 2 diabetes and obesity too. Seems cells that produce insulin in the pancreas are on a circadian schedule too. In terms of obesity risk, disruption of natural circadian rhythms influences hormones that regulate your metabolism. Appetite hormones may also be affected by the circadian disruption. It’s already well known that sleep disruptions cause changes in leptin and ghrelin, hormones that regulate appetite. When circadian rhythms are altered by lack of sleep or lack of light exposure during the day, a host of physiological changes occur that may impact body weight.

This study suggests that exposing your eyes to bright light early in the morning may help with weight control. Other research shows the importance of not exposing your eyes to light at night, during times you would normally be sleeping. This too disrupts circadian rhythms and reduces production of melatonin, a hormone with antioxidant activity that helps regulates the sleep-wake cycle. Low melatonin levels have been linked in animals to an increased risk for certain types of cancer, insulin resistance, sleep problems, and obesity.

Early to Bed, Early to Rise

Although it’s too early to say, not being exposed to bright light early in the morning could cause circadian rhythm changes that affect metabolism and appetite, thereby making it harder to control your weight. Based on the research that’s out there, the best approach is to go to bed as soon after darkness sets in as possible. Yes, I know that’s hard to do! Sleep in complete darkness. If you have to, wear a mask you put over your eyes to block out the light when you sleep. In the morning, open up the curtains as soon as you wake up and soak up the light.

Limit the amount of time you spend on the computer, watching television or on your cell phone at night. These devices emit short wavelengths of light in the blue range that reduce melatonin production by your brain. This makes it hard to sleep, disrupts natural rhythms and could even lead to health problems over time. Special computer glasses are available that block blue light when you wear them.

The Bottom Line

Many aspects of biological function are regulated by your internal biological clock and 24-hour circadian rhythms. Your circadian rhythms are sensitive to light exposure at night and can be disrupted by light at inappropriate times. When your natural rhythms are thrown off, it can alter hormones that regulate your metabolism and appetite. That’s why sleep in general and sleeping in the dark is so important. This new study suggests that getting bright light in the morning, as early as possible, could help with circadian rhythm regulation and weight control, although more research is needed. So get your sleep and open the curtains when you get up.



Medical News Today. “People Exposed to Earlier Sunlight Are Leaner Than Those Who Get Afternoon Sunlight”

Br J Cancer 2004;90:941-943.

Lancet 2 (8498): 89-92. doi:10.1016/S0140-6736(86)91619-3

Science Daily. “Circadian clock in pancreas directly linked to diabetes”

Obes Rev. 2007 Mar;8(2):169-81.

Journal of Pineal Research “Melatonin, energy metabolism, and obesity: a review” 2014

Appl Ergon. 2013 Mar;44(2):237-40.

National Institute of General Medical Sciences “Circadian Rhythms Fact Sheet”


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