An illuminating article on the effects of light on rats Here in the cold north we tend to equate light and health, even if we have discovered sunscreen for ourselves. What about the rats, is light healthy for them? Today's health advice for rats: turn off the light and pull down the blinds, especially for female rats. This article has been published in Gnag, the journal of the Norwegian fancy rat society, as a part of my column "The laboratory rat". The column adresses facts about rats learned from laboratory rats.

You have probably heard that bright light is no good for rat eyes. Now it seems as if too much light is generally bad for the rest of the rat too. A russian team of researchers examined the effect of ”ordinary laboratory light” (12 hours on and off), natural light (northerly!) and 24/7 light on rat health. The results were convincing: The more light, the more tumors, diseases and early ageing. The rats in the study were from LIO strain and were born in the beginning of May 2003.

Diseases and problems more common in the more illuminated groups (natural light and 24/7 light):

• Diabetes
• Irregular estrus in female rats
• Mammary tumors in female rats
• Shortened life span

Blood glucose was registered each month. Abnormal values were visible at 16 months, in 1/5 of the rats in the 24/7 light group. The blood sugar values did not rise nearly as much with age in the 12-12 group as in the other two groups.

The percentage of mammary tumors was roughly twice as large in the 24/7 light group compared to the 12-12 light-and-darkness group.

At eight months all the females in the 12-12 light-and-darkness regular estrus, compared to approximately 10% in the 24/7 light group. At 11 months there were ca. 20% with irregular estrus in the 12-12 light group, but ca. 30% in the 24/7 light group. At 17 months there was a marked difference between rats who were exposed to natural light compared to rats exposed to 12-12 light-and-darkness. The 12-12 rats had 25% cases of irregular estrus whereas the percentage rose to roughly 60% in the natural light group. For comparison the 24/7 group had more than 80% females with irregular estrus.

The groups were fairly small, around 50 rats of each sex in each group. Female rats that were exposed to 12-12 light-and darkness lived on an average around 175 days longer than those with 24/7 light. For male rats the difference was slightly over 60 days. This experiment is not the only one to have found negative health effects of light on rats.

Source: Circadian disruption induced by light-at-night accelerates aging and promotes tumorigenesis in rats Irina A. Vinogradova et al. AGING, October 2009, Vol.1 No.10 pp.855-865