Diabetes and the night-day cycle: What's the link?

Diabetes and the night-day cycle: What's the link?

  • Researchers recently investigated how night-day variations in light intensity affected metabolism in people with insulin resistance.
  • Metabolism was lower during sleep in those exposed to dim light during the day and bright light in the evening.
  • The timing and intensity of light that humans experience daily has subtle effects on glucose and fat metabolism that can contribute to obesity and metabolic diseases.

More than 1 in 3 adults in the United States may have prediabetes, according to the National Institutes of Health.

Insulin resistance is a disorder of metabolism that precedes prediabetes. If left unchecked, prediabetes can develop into type 2 diabetes.

Recently, a team of researchers theorized that societal changes such as increased exposure to artificial light might be one factor linked to the global rise in people with insulin resistance.

As reported in the journal Diabetologia, Prof. Patrick Schrauwen, Jan-Frieder Harmsen, and their colleagues tested their hypothesis by studying individuals who already had insulin resistance.

Insulin resistance occurs when cells do not respond to insulin as efficiently, reducing their ability to take in glucose from the blood.

With painstaking measurements, the researchers were able to determine if varied light exposures altered human metabolism.

What is insulin resistance?

Carbohydrates are complex sugar molecules that people consume through their diet. The body converts these into “simple” sugars in our blood, namely, glucose.

Insulin is a hormone that helps bring glucose from the blood into cells. The cells then convert glucose into an energy-storing molecule called ATP.

If cells are resistant to insulin, glucose remains in the bloodstream, causing a condition called hyperglycemia — elevated blood glucose. Doctors consider this a precursor to developing type 2 diabetes.

Individuals who develop insulin resistance may:

  • consume high calorie diets that are high in fats and sugars and low in fiber
  • have an accumulation of centralized body fat
  • have a genetic predisposition to insulin resistance
  • have obesity or be physically inactive

Scientists suggest that there has been a rise in insulin resistance concurrent with the advent of the industrial revolution fostering a less nutritious diet and decreased physical activity. Insulin resistance is a known risk factor for type 2 diabetes, high blood pressure, and heart disease.

In the recent study, volunteers tested positively for any one of four common parameters that determine insulin resistance, such as an inability to normalize their blood glucose after consuming a sugar-laden drink.

‘Cue the lights’

The scientists based their new research on prior work that confirmed that light is a “zeitgeber,” an important environmental cue that controls the body’s circadian rhythms.

Light cues humans via a specialized brain structure called the suprachiasmatic nucleus, located in the pineal gland.

Dr. Victoria Salem, a senior clinical lecturer in bioengineering and an honorary consultant in diabetes, endocrinology, and internal medicine at Imperial College London, explained to Medical News Today:

“We all have natural body clocks. For example, babies often wake up earlier in the summer months when sunlight pours into the room. Light hitting the back of our eyes is sensed by the brain and our ‘master clock,’ which then sends signals via the nervous system or hormones to other parts of the body, such as the liver or muscle, to regulate digestion and the way we use fuel.”

“For example, the body becomes ‘primed’ in the morning to consume and metabolize the first meal after a long time without food. […] Related to this body clock is the release of the hormone melatonin. The production and release of melatonin are connected to the time of day — increasing when it’s dark and decreasing when it’s light.”

Specifically, the authors of the new study focused on how cycling bright and dim light affected people by measuring:

  • frequent blood glucose levels
  • insulin levels
  • levels of melatonin, a hormone produced by the body to promote healthy sleep
  • fats in the blood known as triglycerides
  • daily energy expenditure
  • sleeping metabolic rate

Life in a metabolic bubble

The researchers were able to make these measurements by asking volunteers to stay overnight, twice, in a respiration chamber while wearing an activity monitoring watch.

The special chamber allowed scientists to measure the amount of oxygen consumed and carbon monoxide produced, giving an accurate measure of the study participants’ metabolic rate.

The participants all ate similar meals and performed similar physical activities.

Next, the researchers exposed 14 participants to two variations of daylight and evening: 10 hours of bright light followed by 5 hours of dim light in the evening — a “normal” human day — or 10 hours of dim light during the daytime and 5 hours of bright light before sleep.

What they found

In these carefully controlled circumstances, the scientists discovered several interesting results:

  • Melatonin release was significantly supressed in the dim day-bright evening scenario.
  • Those who spent the day in bright light had lower glucose levels before dinner and higher metabolic rates at night.
  • Metabolism slowed after dinner in individuals in the dim day-bright evening scenario.
  • In people who spent the day in bright light with a dim night, melatonin levels were increased at night, which is helpful for inducing sleep.

The researchers concluded that the timing of light exposure can influence the body’s handling of glucose and fats, energy use and expenditure, and even temperature regulation in insulin-resistant individuals.

Natural Iight cycles are important to metabolism

Dr. Victoria Salem summarized the findings for MNT:

“The more natural light conditions were associated with subtle improvements in certain metabolic parameters. [M]ost interestingly, it aided a higher metabolic rate at night — good for burning calories!”

“This was associated with normal melatonin release compared to its inhibition with bright evening light,” Dr. Salem continued.

“The effect described here is probably small, but nonetheless important, since a holistic approach to improving well-being — which includes, amongst other things, healthy lighting, keeping the room cool at night, and good sleep hygiene — is rather good advice.”

The authors of the study recommend further research in two realms: more investigation into how light exposure affects energy expenditure and studies into how thoughtful lighting control in the home and office settings may improve health outcomes.

MNT asked Dr. Victoria Salem if any research supports the theory that sleeping better or natural light patterns can reduce metabolic disruptions. She replied:

“Yes, but not high quality [randomized controlled trials], and since the effect size is small it is going to be difficult to prove clinically. Having said that, I do believe that this study is telling us something important — that as part of a holistic approach to good metabolic health and general well-being, encouraging good sleep and evenings where we wind down and reduce bright screen time is a very good idea.”

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