We tend to think of weight gain as a slightly complicated equation of diet, exercise and hormones. But there’s another important factor we might be ignoring: our relationship with light.
In her latest book, How to Age, Liz explains that light is far more than something that helps us see. The timing, brightness and type of light we’re exposed to acts as vital information for our biology. It helps to regulate everything from our sleep and appetite to metabolism and energy production.
“When our circadian rhythm runs smoothly, our metabolism works efficiently, inflammation stays lower and we’re more likely to maintain a healthy weight,” explains Liz.
Here, we explore the science behind light and metabolism, and what paying closer attention to our body’s natural rhythms could mean for our weight, energy and appetite.
Leptin and light
Dr Sara Pugh, PhD began her career as a biochemist, where she used to think metabolism and weight were solely a food and exercise matter. But, with the rise in obesity, and weight loss being a multi-billion pound global market, she began to wonder whether something was missing from the conversation. “Most people think weight gain is about calories alone,” she says. “It’s not.”
As Sara explains, body fat is regulated by the brain – specifically the leptin-melanocortin pathway in the hypothalamus. “Leptin is a hormone, released from fat cells, that tells the brain whether to use energy or store it as fat,” she says.
Liz describes leptin as the body’s ‘fuel gauge’.
“When functioning properly, it tells the brain that we’ve had enough to eat and have sufficient energy reserves,” she says. “Leptin doesn’t act in isolation. It’s closely linked to our circadian rhythm and highly sensitive to light.”
Energy is everything
For Sara, leptin is really an electromagnetic energy accountant, and light is part of the electromagnetic spectrum that provides energy and information to sustain life.
“When leptin works, it activates our POMC pathway (aka, fat burning) and suppresses our hunger and fat storage. Leptin also controls thyroid hormones, which are our metabolic accelerator pedal – this is how we can stay lean without forcing it.”
Sara tells us that when leptin doesn’t work, the brain thinks we’re starving – even if we’re overeating or overweight. “That’s called leptin resistance,” she explains. “And it’s not primarily caused by food. It’s caused by light.”
Liz explains that when we receive too little morning sunlight, or too much artificial light after dark, leptin signalling can become disrupted.
“Leptin resistance is where the brain no longer ‘hears’ leptin’s message,” she says. “This increases hunger, reduces energy expenditure and encourages fat storage.”
The right light
Not all light is created equal. It’s not as simple as blasting yourself with light at some point in the day and watching the pounds magically fall away. Timing is everything.
Research published in the journal Nutrients looked at how the properties of light – intensity, duration, timing and wavelength – can affect metabolism through circadian rhythms.
The researchers found that bright light during the day, particularly in the morning, may support healthy metabolism, but light at the wrong time – especially at night – can be harmful. The study suggests that circadian disruption is a driver of obesity and metabolic disease. It’s less about how much light we get, and more about when.
“When our circadian rhythm runs smoothly, our metabolism works efficiently, inflammation stays lower and we’re more likely to maintain a healthy weight,” says Liz.
Get back in sync
When this rhythm is disrupted by late nights and prolonged exposure to artificial light, mitochondrial function may suffer. This makes weight gain and metabolic dysfunction more likely.
As Sara reveals, our brain is wired to use light as the master signal for timing biology. “Sunrise, morning sunlight and ultraviolet light sets your circadian rhythm, aligns hormones like cortisol, insulin and dopamine, and keeps the hypothalamus sensitive to leptin,” she shares.
But modern life flips this completely. Days spent indoors under artificial blue light and LED screens can impact sleep and impair dopamine signalling, making evening snacking on high-energy food more likely. “The result is a brain that can’t respond to leptin,” Sara explains. “Artificial light damages the leptin–melanocortin pathway.”
It’s not just leptin that’s affected. Liz highlights another hormone called ghrelin, often referred to as the ‘hunger hormone’. Ghrelin helps stimulate appetite before meals, but its natural rhythm can also be disrupted by poor sleep and artificial light exposure, potentially increasing cravings for sugary and high-calorie foods.
“When leptin and ghrelin fall out of sync, the result is often increased appetite, slower metabolism and a higher likelihood of weight gain,” says Liz.
Fix your environment
Sara warns that even if leptin levels are high, artificial blue light can prevent the system from working as it should. What does this look like in practice? Hunger increases, metabolism slows and fat storage rises. “This is why dieting can fail, or why weight keeps coming back even when you eat less,” Sara explains.
But blue light isn’t necessarily the enemy; it comes down to how and when we’re exposed to it.
Dawn light is naturally rich in blue wavelengths. A study published in Nature found that sunlight – specifically its blue wavelength component – can pass through the skin and directly stimulate fat cells via a light-sensing protein previously thought to exist only in the eye. The researchers discovered that these visible blue wavelengths physically penetrated the skin and triggered fat cells to release and break down lipids.
Growing evidence shows that sunlight may influence weight regulation through multiple pathways. Emerging research suggests that light exposure may affect brain chemicals involved in appetite control and energy expenditure. Red and near-infrared light appear to support mitochondrial function, helping cells produce energy more efficiently.
What can we do to support our metabolism?
As Liz explains, supporting healthy metabolism is less complicated than we think.
“Natural sunlight, red-light therapy and keeping evenings and bedtime dark will all support our metabolism,” she says. “They’ll also help keep our bodies in a fat-burning, anti-inflammatory state by boosting mitochondrial function and enhancing both leptin and insulin sensitivity.”
Liz also suggests only eating in daylight hours where possible and reducing exposure to blue light after sunset by wearing blue-light-blocking glasses.
For Sara, the answer is equally straightforward. Get outside early and seek plenty of bright natural light in the morning. In the evening, reduce or remove artificial light. These simple steps can meaningfully improve leptin signalling.
“This is one of the most crucial dualities in health,” she says. “When leptin works, the brain stops guarding fat. And because leptin regulates not just weight, but bone density, fertility and the immune system, it is so much more than just a ‘fat hormone’.”
While scientists are still uncovering exactly how light influences metabolism, appetite and fat storage, one thing is becoming increasingly clear: sunlight may be a far more important part of healthy ageing and weight management than we’ve previously given it credit for.
