Bright indoor light during daytime may lower blood sugar, improve energy expenditure

According to a new study published in Diabetologia, Insulin-resistant volunteers' postprandial substrate processing, energy expenditure, and thermoregulation are all affected by the indoor light environment in a time-dependent manner.Further Optimization of indoor lighting to a brighter during daytime hours and dimmer in the evening may provide cardiometabolic benefits.

Artificial light is available 24 hours a day in today's civilization, and most individuals are exposed to electrical light and light-emitting screens during the dark part of the natural light/dark cycle. Suboptimal lighting has been linked to negative metabolic impacts, and changing indoor lighting to more closely mirror the natural light/dark cycle has the potential to improve metabolic health.

This study was conducted by Jan-Frieder Harmsen and team with the objective to evaluate metabolic reactions in persons at risk of developing metabolic disorders to lighting conditions that resembled the natural light/dark cycle against poor illumination.

This was a non-blinded, randomized, controlled crossover trial in which 14 obese insulin-resistant volunteers (n = 14) were exposed to two 40-hour laboratory sessions with different 24-hour illumination regimens while remaining in a metabolic chamber under real-life settings. Volunteers were exposed to electric bright light (1250 lx) during the day (08:00–18:00 h) and dim light (5 lx) during the evening (18:00–23:00 h) in the Bright day–Dim evening condition. Volunteers were exposed to dim light during the day and strong light during the evening in the Dim day–Bright evening condition. Sequential numbering was used for randomization and assignment to light conditions. 24 hour indirect calorimetry, continuous core body and skin temperature readings, and regular blood samples during both lighting treatments were done. Plasma glucose levels were measured during the pre- and postprandial periods of the intervention as the primary outcome.

The key points of this study were as follow:

1. When compared to dim light, spending the day in bright light resulted in a higher increase in postprandial triacylglycerol levels after breakfast but lower glucose levels before the dinner meal at 18:00 h.

2. When compared to Bright day–Dim evening, Dim day–Bright evening lowered the increase in postprandial glucose after dinner.

3. The sleeping metabolic rate was identical to the baseline night after the Bright day–Dim evening condition, while it reduced after the Dim day–Bright evening condition.

4. For Dim day–Bright evening, melatonin secretion was considerably reduced, but not for Bright day–Dim evening.

5. When compared to Dim day–Bright evening, the distal skin temperature for Bright day–Dim evening was lower at 18:00 h and higher at 23:00 h.

6. The respiratory exchange ratio and fasting and postprandial plasma insulin levels were not different between the two illumination schemes at any time.

In conclusion, the fact that in the Dim day–Bright evening condition, postprandial TG levels after breakfast and postprandial glucose levels after supper were less elevated needs additional investigation. More research is needed in the future to see if changing light regimens in office buildings and at home can help prevent metabolic illnesses.

Reference:

Harmsen, JF., Wefers, J., Doligkeit, D. et al. The influence of bright and dim light on substrate metabolism, energy expenditure and thermoregulation in insulin-resistant individuals depends on time of day. Diabetologia (2022). https://doi.org/10.1007/s00125-021-05643-9