Nov. 8, 2005 — Bright light is known to affect the body and its internal “clock,” and Japanese scientists may have partly figured out how that happens.

When they exposed mice to bright light, the mice experienced a wave of hormones called glucocorticoids. These hormones are responsible for many bodily processes including metabolism, response to stress, inflammation, and immunity.

Atsushi Ishida and colleagues report their findings in Cell Metabolism. Ishida works in Kobe, Japan, in the brain science department of Kobe University’s medical school.

The study doesn’t change the use of light therapy in people for conditions including sleep disorders and some types of depression, such as seasonal affective disorder (SAD). But it might explain one aspect of how light therapy works.

From Darkness to Bright Light

Ishida’s team did a series of tests on mice. In one experiment, mice were briefly kept in constant darkness and then exposed to a short session of bright light.

Next, the researchers checked the genes in the mice’s adrenal glands. Located atop the kidneys, the adrenal glands make glucocorticoid hormones.

Exposure to light boosted gene activity in the mice’s adrenal glands. That upped production of hormones made by the adrenal glands. Those hormones could then travel throughout the body, docking on virtually any cell to rev up cell metabolism.

The intensity of the light determined the size of the hormonal response. Very intense light prompted a bigger hormonal surge, the study shows.

Brain Link

The process started in the mice’s brains, the researchers report. They focused on a brain area that’s deeply involved in the internal “body clock.”

That brain area is called the suprachiasmatic nucleus.

In another test, researchers took the suprachiasmatic nucleus out of the loop. When that happened, the mice didn’t show the same hormonal reaction to bright light.

What About People?

The tests weren’t done on humans. If the results apply to humans, it could be “of great physiological interest” for doctors and researchers, editorialists write.

They point out that it would be pretty easy to check hormone levels after light exposure. They also note that the findings might explain light therapy’s benefits for SAD patients and those with other types of depression that aren’t usually associated with the internal clock.

The editorialists included Ueli Schibler, who works in Switzerland in the University of Geneva’s molecular biology department. Schibler and colleagues weren’t involved in Ishida’s experiments.