Northwestern University scientists studying the interconnection of the circadian rhythm (our biological clock) and light sensitivity have found that the body is more insulin resistant when exposed to bright light, as opposed to dim light, in both the morning and the evening.
The researchers also found that bright conditions in the evening deregulate metabolic enzymes, causing higher peak glucose. Over time, the build-up of blood sugar can result in increased body fat and weight gain, while in the short term puts late night eaters at higher risks of seeing glucose not clearing out of the bloodstream as fast later in the evening.
The capacity of varying amounts of light to influence metabolism function and help regulate weight is well-known but how critical when in the day you get the majority of your light exposure was unclear, until these new findings were published in the Journal PLOS ONE.
A previous research study conducted by the same group of researchers from the Northwestern University Feinberg School of Medicine in Chicago in 2014 partially established that specific timing and intensity of light correlate with metabolic health.
The team examined the impact different lightning environments had on 54 people with normal body mass indexes (BMIs) in terms of adverse health outcomes and their ability to maintain their weight.
The participants who were exposed to the brightest levels of light (500 lux), starting at 8 am, had significantly lower BMIs than those soaking up that much light later in the day.
In the Northwestern University’s latest work, the three-hour blue-enriched light treatment that 19 healthy adults received upon waking and before bed isn’t presented as an independent predictor of biological markers associated with metabolic health. The research team adjusted for other variables that could factor into this relationship, such as their calorie intake, activity levels, sleep patterns and hormone production.
One hypothesis researchers don’t rule out for why exposure to light at particular times of the day improves or deteriorates metabolism functions has to do with the simultaneous interaction between light, our internal clock, and a key hormone activated during sleep governing it, melatonin.
Transition from dim to bright light in the morning also induces an immediate elevation of cortisol levels, which we know is tightly linked to insulin secretion.
Fixed meal times have been observed by all participants twice a day in the light for the sake of the experiment, but nothing is said about the caloric density or glycemic index of the food ingested on those occasions, which can greatly influence insulin requirements and glucose management, even more so than light itself.
The studies have show, however, that altering light exposure too dramatically later in the day changes hormones that regulate appetite like leptin and ghreli, and when we eat. Number of studies have reported that people who go to bed later tend to consume more calories and make more unhealthy food choices, exacerbating pre-existing insulin sensitivity.
Instead, recommend the researchers, we should try consciously to be exposed to light during the day, and the earlier the better. In one clinical study, for instance, obese women saw their body fat drop after three weeks of 45 minutes of light exposure between the hours of 6 a.m. and 9 a.m.

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