Late-night bright light exposure shifts metabolism away from fat burning Original paper

Exposure to artificial light at night has been associated with a higher body weight and a higher risk of obesity, but the reason for this relationship is unclear.^[1] One possibility is that light exposure before sleep may result in a disruption of melatonin signaling and the circadian rhythm, leading to changes in metabolism that predispose people to fat gain.

This randomized controlled crossover trial examined the effect of exposure to light at night on energy metabolism. A total of 10 men (mean age of 26) were assigned to either a bright light at night condition (10,000 lux) or a dim light condition (<50 lux) between 9:00 PM and 12:00 AM, with the participants sleeping from 12:00 to 7:00 AM. The participants were assigned in random order to each condition and later crossed over to the other condition after at least 7 days.

The participants slept in a metabolic chamber, which allowed the investigators to measure their energy expenditure, respiratory quotient (RQ), and oxidation of carbohydrates, fat, and protein. The investigators also assessed the participants’ salivary melatonin (morning and evening) and several sleep parameters (sleep latency, wake after sleep onset, total sleep time, and sleep efficiency).

Compared to the bright light group, melatonin levels were higher in the dim light group in the two hours before bedtime. The area under the curve (AUC; a measure of systemic exposure) for melatonin levels was 63% lower in the bright light group, compared with the dim light group, between 9:00 PM and 12:00 AM.

Compared with the dim light group, the bright light group experienced an increase in RQ during light exposure and during sleep, a decrease in fat oxidation during light exposure and during sleep, and an increase in carbohydrate oxidation during sleep.

There were no differences between groups for any of the sleep parameters.