Extended exposure to light during nighttime can have negative consequences for human health. But now, researchers from University of Tsukuba have identified a new type of light with reduced conse­quences for physio­logical changes during sleep. They compared the effects of light-emitting diodes (LEDs), which have been widely adopted for their energy-saving pro­perties, with organic light-emitting diodes (OLEDs) on physical processes that occur during sleep.

Polychromatic white LEDs emit a large amount of blue light, which has been linked with many negative health effects, including meta­bolic health. In contrast, OLEDs emit polychromatic white light that contains less blue light. However, the impact of LED and OLED exposure at night has not been compared in terms of changes in energy metabolism during sleep, something the researchers aimed to address. “Energy metabolism is an important physio­logical process that is altered by light exposure,” says Kumpei Tokuyama. “We hypo­thesized that compared with LEDs, OLED exposure would have a reduced effect on sleep archi­tecture and energy metabolism, similar to that of dim light.”

To test this hypothesis, the researchers exposed 10 male participants to LED, OLED, or dim light for 4 hours before they slept in a metabolic chamber. The researchers then measured energy expen­diture, core body temperature, fat oxidation, and 6-sulfatoxy­melatoni – which is a measure of melatonin levels during sleep. The participants had not recently traveled or parti­cipated in shift work. “The results confirmed part of our hypothesis,” explains Tokuyama. “Although no effect on sleep architecture was observed, energy expenditure and core body tempera­ture during sleep were significantly decreased after OLED exposure. Furthermore, fat oxidation during sleep was signi­ficantly lower after exposure to LED compared with OLED.”

In addition, fat oxidation during sleep was positively correlated with 6-sulfatoxy­melatonin levels following exposure to OLED, suggesting that the effect of melatonin activity on energy metabolism varies depending on the type of light exposure. “Thus, light exposure at night is related to fat oxidation and body tempera­ture during sleep. Our findings suggest that specific types of light exposure may influence weight gain, along with other physio­logical changes,” says Tokuyama.

Many occu­pations and activities involve exposure to arti­ficial light before sleep. New information about the effects of different kinds of light on physical processes may faci­litate the selection of alternative light sources to mitigate the negative conse­quences of light exposure at night. Further­more, these findings advance our knowledge regarding the role of light in energy metabolism during sleep. (Source: U. Tsukuba)

Reference: A. Ishihara et al.: Metabolic responses to polychromatic LED and OLED light at night, Sci. Rep. 11, 12402 (2021); DOI: 10.1038/s41598-021-91828-6

Link: International Institute for Integrative Sleep Medicine (WPI-IIIS), University of Tsukuba, Tsukuba, Japan