Acute Effects of Morning Light on Plasma Glucose and Triglycerides in Healthy Men and Men with Type 2 Diabetes

Abstract

Ambient light intensity is signaled directly to hypothalamic areas that regulate energy metabolism. Observational studies have shown associations between ambient light intensity and plasma glucose and lipid levels, but human data on the acute metabolic effects of light are scarce. Since light is the main signal indicating the onset of the diurnal phase of physical activity and food intake in humans, we hypothesized that bright light would affect glucose and lipid metabolism. Therefore, we determined the acute effects of bright light on plasma glucose and lipid concentrations in 2 randomized crossover trials: (1) in 8 healthy lean men and (2) in 8 obese men with type 2 diabetes. From 0730 h, subjects were exposed to either bright light (4000 lux) or dim light (10 lux) for 5 h. After 1 h of light exposure, subjects consumed a 600-kcal mixed meal. Primary endpoints were fasting and postprandial plasma glucose levels. In healthy men, bright light did not affect fasting or postprandial plasma glucose levels. However, bright light increased fasting and postprandial plasma triglycerides. In men with type 2 diabetes, bright light increased fasting and postprandial glucose levels. In men with type 2 diabetes, bright light did not affect fasting triglyceride levels but increased postprandial triglyceride levels. We show that ambient light intensity acutely affects human plasma glucose and triglyceride levels. Our findings warrant further research into the consequences of the metabolic effects of light for the diagnosis and prevention of hyperglycemia and dyslipidemia.

Keywords: artificial light; glucose metabolism; lipid metabolism; type 2 diabetes.

Figure 1.

Study design. The healthy subject trial and the type 2 diabetes patient trial were both designed as randomized crossover intervention trials. Subjects were admitted to the clinical research unit twice, with a 1-week interval. Subjects entered the clinical research unit after a 1-h fast at 2000 h. At 2130 h, subjects received a standard 800-kcal mixed meal, and they were allowed to sleep in darkness from 2330 until 0730 h. In the dim light condition, subjects were subsequently exposed to 10 lux emitted by one HF3319 EnergyLight (Philips) placed in the room corner. In the bright light condition, subjects were exposed to 4000 lux bright light emitted by two HF3319 EnergyLights placed in front of the subject. A cannula was inserted in a peripheral arm vein at 0800 h, and frequent blood samples were obtained beginning at 0815 h. At 0830 h, participants consumed a 600-kcal liquid mixed meal. In the type 2 diabetes patient trial, additional measurements were performed: saliva samples (asterisk) were obtained at regular intervals, and hunger, prospective food consumption, fullness, and satiety were assessed with a questionnaire.

Figure 2.

In healthy men, bright light did not affect plasma glucose levels but increased fasting and postprandial triglyceride levels. (A) Fasting and postprandial glucose levels were not different between bright light and dim light. (B) Insulin levels were not different between bright and dim light. (C) C-peptide levels were not different between bright and dim light. (D) Fasting and postprandial triglyceride levels were elevated due to exposure to bright light compared with dim light. (E) Free fatty acid levels did not differ between bright light and dim light. (F) Heart rate did not differ between bright light and dim light in healthy men. (G) LF:HF ratio was slightly increased in bright light compared with dim light. Data are shown as mean ± SEM of 8 subjects per group. Asterisks indicate p < 0.05. Open circles are bright light and closed circles are dim light. The dashed line represents mealtime, and the shaded area indicates lights-off.

Figure 3.

In men with type 2 diabetes, bright light increased fasting and postprandial glucose levels and postprandial triglyceride levels. (A) Fasting and postprandial glucose levels were elevated due to exposure to bright light compared with dim light. (E) Postprandial triglyceride levels were also elevated due to bright light. However, fasting and postprandial (B) insulin, (C) C-peptide, (D) glucagon, and (F) free fatty acid levels did not differ between bright and dim light. (G) Heart rate was increased in bright light compared with dim light. (H) LF:HF ratio was slightly increased in bright light compared with dim light. Data are shown as mean ± SEM of 8 subjects per group. Asterisks indicate p < 0.05. Open circles are bright light and closed circles are dim light. The dashed line represents mealtime, and the shaded area indicates lights-off.

Figure 4.

In men with type 2 diabetes, bright light increased appetite scores. During fasting and 5 h postprandially, (A) hunger and (B) prospective food consumption scores were elevated in bright light compared with dim light. (C) Fullness and (D) satiety scores were significantly decreased in bright light compared with dim light 5 h postprandially. Data are shown as mean ± SEM. Asterisks indicate p < 0.05. Open circles are bright light and closed circles are dim light. The dashed line represents mealtime, and the shaded area indicates lights-off.