The research confirmed the hypothesis that the light during sleep brings the development of obesity, stimulates various diseases, including cancer, and reduces life expectancy.

For the experiment, the researchers divided a thousand newborn rats into four groups. The first lived all their life in the natural light of Petrozavodsk with white nights in summer and night illumination of streets in winter. The second group lived in constant light, the third in the dark, the fourth in the "equatorial" light regime of 12 hours of light and 12 hours of darkness. All the other conditions were usual for rats: food, exercises (jogging, swimming, walking through the labyrinths). The animals were regularly tested. After natural death an autopsy was performed to detect diseases.


More than half of the animals of the first group were overweight and showed the worst results of all four groups. Scientists refer to the large rats weighing over 440 grams (normal weight for adult rats is 250 to 440 grams). In addition to the large weight, this group of test animals had high level of cholesterol and blood glucose and decreased insulin levels. These are signs of the metabolic syndrome as a risk factor for cardiovascular disease and diabetes.