fat buddha illustrating obesity, fat, circadian metabolism syndrome

Is obesity related to something as distant to fat on our bellies as the sunshine, lamp-shine and display-shine? No matter how weird it sounds, science has repeatedly shown the connection.

Circadian disruption results in desynchronization among neurons of the SCN, which can decrease the circadian rhythm amplitude and eventually lead to metabolic disorders such as obesity and insulin resistance.

Coomans et al. (2015). Plasticity of circadian clocks and consequences for metabolism . Diabetes, Obesity and Metabolism 17 (Suppl. 1): 65–75.

Obesity: the timing matters

In 2010, scientists were checking genes of people undergoing treatment at Spanish obesity clinics. Specifically, they were interested in a gene called CLOCK, which is responsible for our circadian rhythms. And they confirmed a surprising finding of previous studies: some versions of this gene helped reducing weight while others made it more difficult. The effect was quite large: the single gene caused difference of 2 kilograms during few months of treatment. Moreover, the effect was not only related to obesity itself, but also to cholesterol levels.

How can a gene responsible for time-keeping influence obesity? The cause lies in the timing of our bodily functions. In the course of day and night, levels of the substances circulating in our blood and affecting our brains and guts fluctuate. And when misaligned, the guts don’t synchronise with our activity – resulting in excessive food processing.

Within the brain, it is the dopaminergic system what motivates us to seek rewards, including food – and in well-functioning organism, dopamine production is reduced for the night. Together with a molecule called hypocretin affecting our overall activation and behaviour, including food-seeking.

Outside the brain, insulin is regulated by the natural circadian rhythm, too. In healthy organism adapted to the natural cycle of days and nights, all these molecules are produced predominantly during the day. However, life in the modern society, full of irregularities and delays of bedtime, may easily disrupt this synchronization, adding up to other factors causing obesity.

Delaying the habitual sleep phase reduces insulin sensitivity and enhances a marker for inflammation, presumably by exposing peripheral organs to nutrients during the habitual overnight fast period.

Borberly A. et al. (2016). The two-process model of sleep regulation: a reappraisal. J Sleep Res. 25, 131–143.

The proof is in the eating. Literally

But in the beginning, we only had the CLOCK genes correlating with obesity. Isn’t it the genes themselves what is causing the propensity for obesity and metabolic syndrome rather than the circadian rhythm?

No, it is the rhythm itself! The same effect has been proven not only in mice with disrupted clock gene – but also in mice with the natural rhythm disrupted by light. Mice fed with high-fat diet had twice more fat and twice higher fat-to-lean mass ratio when their circadian rhythm was disrupted by constant bright light compared to the natural rhythm of light and darkness.

Finally, these findings have been further confirmed by a study showing that lesion in the SCN, the part of hypothalamus in the brain where our inner clock resides, results in the same problems: insulin resistance and obesity.

Of course, this does not mean that lacking or excess light would make you obese or the metabolic syndrome would be caused by circadian disruption. There are many other factors adding up, resulting in the weight and risks you have. But it has been proven that the circadian rhythm governed by light plays a role as well.

Light disruption and obesity epidemy: cause and effect or coincidence?

Our society suffers from obesity and metabolic disorders – while we are increasingly exposed to light smog at night. Are these two facts related?

Let us start with animals: Mice normally live – and eat – at night. But when their daylight was dim, this rhythm is disrupted and they eat half more food during the day. In another study, researchers kept the natural light same for all mice – but fed some of them during the day and some of them at night. Results: the mice fed in their natural sleep phase ate a bit more, exercised a bit less – and grew much fatter. While those eating at natural times kept their healthy 25 grams of weight, the ones fed at inappropriate times exceeded 30 grams on average.

In humans, the research is more sophisticated because we cannot simply lock a bunch of innocent guys in a lab and expose them to light at random intervals or feed them only at nights. However, even in humans we know that the hyperglycaemia caused by circadian disruption can reach pre-diabetic levels. As of 2019, scientists theorize that the disruptions of daily rhythm (so called circadian syndrome) play much more important role in obesity, diabetes and cardiovascular disease than we thought ever before:

With the increased recognition of the ‘Circadian Syndrome’, circadian medicine, through the timing of exercise, light exposure, food consumption, dispensing of medications and sleep, is likely to play a much greater role in the maintenance of both individual and population health in the future.

Zimmet, Alberti, Stern, Bilu, El-Osta, Einat, Kronfeld-Schor (2019). The Circadian Syndrome: is the Metabolic Syndrome and much more! Journal of Internal Medicine. 286(2):181-191

Can we treat obesity with light?

The answer is closer than it looks: some studies already recommend bright light treatment as a part of weight control procedures

Increased appetite, carbohydrate craving and weight gain are signs of the so-called atypical depression, also common in seasonal affective disorder (SAD). And SAD can be efficiently treated by bright light lamps mimicking the sunshine. In an old study devoted specifically to these atypical symptoms back in 1990, weight gain dropped from 60% to 12% , carbohydrate craving from 47% to 20% and excessive eating from 45% to 9% in those suffering from SAD. And the clinical evidence from obesity studies is promising as well.

In 2007, a group of Canadian scientists split 25 overweight and obese participants into two groups. While all of them were exercising for 6 weeks, only one group also received bright light therapy devices and instructions to use them in the morning. While there was no difference proven between the groups in weight and BMI, the body fat composition slightly, yet significantly improved in the bright light group compared to the other one.

Similar findings were presented in 2013 from Novosibirsk, Russia: obese women exposed to bright light device lost almost a pound (0.35kg) of fat compared to the control group. It is noteworthy that both studies were conducted in high latitude over the winter period. Of course, artificial light exposure may be of use for those who lack the natural one.

Morning bright light treatment reduces body fat and appetite in overweight women and may be included in weight control programs.

Danilenko K.V. · Mustafina S.V. · Pechenkina E.A. (2013). Bright Light for Weight Loss: Results of a Controlled Crossover Trial, Obes Facts; 6:28-38.

Even in higher latitudes, however, light treatment is unlikely to significantly help everyone. The original study from 1990 might serve as a great example: Despite the described effects in SAD patients, the same values were much more conservative in a group of (non-seasonally) depressed people. Nevertheless, the presented studies clearly show the capability of bright light therapy to help at least some of us, even with a problem as distant from light perception as obesity is.


This was the last article from our Science Behind Light Therapy series. Understanding the neural underpinnings, you can appreciate the studies evaluating light therapy in various particular disorders. For research proving the effectiveness of light therapy, you may visit our section devoted to meta-analytical studies.