Optimizing indoor lighting to be brighter during daytime hours and dimmer in the evening may provide cardiometabolic benefits, according to study findings published in Diabetologia.

In findings from a randomized controlled trial, participants who were in an indoor environment with bright lighting during the day and dim lighting in the evening had lower plasma glucose levels and increased in energy expenditure compared with bright lighting in the evening and dim lighting during the day, providing evidence that indoor lighting should correspond to the natural day-night cycle.

“Insulin resistant, prediabetic people have disturbances in their 24-hour energy and substrate metabolism and circadian clock, which may contribute to the development of diabetes,” Patrick Schrauwen, PhD, professor of metabolic aspects of type 2 diabetes in the NUTRIM School of Nutrition and Translational Research at Maastricht University in the Netherlands, told Healio. “These may be due to — among other factors — suboptimal light conditions, as many people spent most of their times indoors. Optimizing the light condition may be able to reset some of these metabolic disturbances.”

Schrauwen and colleagues conducted a randomized controlled crossover trial with 14 adults aged 40 to 75 years with overweight who met one of four criteria for insulin resistance. Only participants with a bedtime around 11 p.m. and a sleep duration of 7 to 9 hours were enrolled. Participants stayed in a respiration chamber for two seperate 40-hour periods beginning at 6 p.m. on day 1 and ending at noon on day 3. In one of the sessions, participants were exposed to bright lighting during daytime from 8 a.m. to 6 p.m. and dim lights during the evening from 6 p.m. to 11 p.m. In the other session, separated by a washout of at least 4 days, participants were in a dim lighting environment from 8 a.m. to 6 p.m. and bright lights from 6 p.m. to 11 p.m. Volunteers wore an actigraph to measure sleep patterns before and during the study. Energy expenditure, sleep metabolic rate and substrate oxidation were calculated based on oxygen consumption and carbon dioxide production. Wireless sensors were used to measure skin temperature. Fasting blood samples were collected at 7:45 a.m. on days 2 and 3 and at 5:45 p.m. on day 2, and postprandial blood samples were taken every 30 minutes for 4 hours after breakfast each day and dinner on day 2.

Lower glucose with bright daytime lighting

There were no changes in proximal skin temperature between the two conditions, but distal skin temperature was lower at 6 p.m. for the bright lighting during the day pattern compared with spending the day in dim lighting (28.8°C vs. 29.9°C; P = .039). At 11 p.m., those in the bright day light pattern had a higher distal skin temperature compared with those in the bright evening light condition (30.1°C vs. 28.8°C; P = .006).

Participants in the bright day lightning pattern had a greater increase in plasma triglycerides after breakfast on day 2 compared with dim lighting during the day (P = .029). Plasma glucose was lower just before dinner for those in the bright day light pattern compared with bright light in the evening (5 mmol/L vs. 5.2 mmol/L; P = .02).

Energy expenditure improved with bright daytime lighting

Energy expenditure on day 2 and day 3 was not significantly different between lighting patterns. There was also no difference in energy expenditure in the postprandial phase of the day 2 breakfast. After dinner, those in the bright day light condition had greater postprandial energy expenditure compared with bright evening lighting. Sleeping metabolic rate was lower in the night after the light intervention compared with before the bright light intervention in the bright evening light pattern only. Respiratory exchange ratio did not differ between the conditions in any time interval.

“Optimizing light conditions, including bright light during the day and dim light during the night, affects 24-hour metabolism in humans and may be important to improving metabolic health in individuals at risk for developing type 2 diabetes or insulin resistance,” Schrauwen said.

However, Schrauwen noted that the study cohort was small and the study duration short at 40 hours. He said longer studies with more participants are needed to determine whether the light conditions may translate into clinically relevant improvements, and these first results justify such further studies.