New research shows that experiencing less daylight during winter changes mice’s eating patterns and metabolism . A researcher says that this discovery could influence experiments with animals involving drugs for obesity or research on diabetes.
Winters can be dark far to the north and south, with the night-time hours vastly exceeding the daylight hours.
Scientists have long known that people’s circadian rhythms affect their metabolism. For example, people who work at night or often experience jet lag have a greater risk of having overweight or developing metabolic disorders such as type 2 diabetes.
Now a new study shows that the number of hours of light mice experience per day (the photoperiod) affects their eating patterns and changes their metabolism.
The discovery may provide greater insight into how people’s photoperiod affects their weight and indicates the importance of considering the photoperiod in scientific experiments with animals.
“Research is lacking on how changes in the quantity of daylight people experience in summer and winter may affect how they develop type 2 diabetes, obesity and metabolism. Most studies of the effects of seasonal changes in the photoperiod have been carried out on animals that naturally hibernate and thereby change their metabolism, but in this study we investigated the effects on mice. They do not hibernate and are used in many experiments worldwide, almost exclusively experiencing a photoperiod of 12 hours of light each day. This can potentially affect their metabolism and thus the result of specific experiments, which are also carried out to learn more about how people would be affected,” explains a researcher behind the study, Lewin Small, a Postdoctoral Fellow who performed the research at the Novo Nordisk Foundation Center for Basic Metabolic Research at the University of Copenhagen.
The research has been published in Cell Metabolism.
Changing mice’s photoperiod
Light affects animals and people in all aspects of life, including regulating our sleep, hormonal cycles and brains.
Lewin Small and colleagues wanted to determine whether the photoperiod influences mice’s metabolism and how. They therefore exposed mice to three different photoperiods.
One group experienced 6 hours of light per day, approximately corresponding to the quantity of daylight at the latitude of Denmark in winter. The second group experienced 12 hours of light and 12 hours of darkness, corresponding to the quantity of daylight at the equator year-round and at the spring and autumn solstices in the rest of the world. The last group experienced 18 hours of light per day, corresponding to the quantity of daylight around the summer solstice at the latitude of Denmark.
The researchers investigated how the different photoperiods affected the mice’s weight, eating patterns, activity and blood glucose over 12 weeks.
“In many previous experiments, we have used mice to study how the circadian rhythm influences the body, including metabolism. We already knew that the photoperiod affects mice,” says Lewin Small.
The photoperiod affects mice’s metabolism
The results show very clearly that the photoperiod strongly influences mice’s metabolism.
The mice experiencing a photoperiod equivalent to winter in Denmark gained less weight than mice experiencing a summer photoperiod.
Their food intake also differed. Mice experiencing more darkness ate one large meal and fasted for much of the rest of the day, whereas mice experiencing more light balanced their food intake more evenly throughout the day.
Mice experiencing a short photoperiod also had higher lipid metabolism and reduced liver triglycerides.
However, the difference between the mice evened out when the researchers only allowed the mice to eat at one specific time each day.
The mice experiencing more darkness were more active and used more energy, which makes sense since mice are nocturnal.
The researchers also investigated whether melatonin, a hormone that strongly influences sleep, triggered the difference between mice differing in photoperiod. However, these experiments involving mice engineered so that they cannot make melatonin showed that melatonin does not play a major role.
“We arrived at the same result, and this suggests that melatonin does not drive the difference in metabolism in mice experiencing short and long photoperiods. The quantity of light affects activity and food intake,” explains Lewin Small.
People have largely engineered their photoperiods
Lewin Small explains that these observations probably have an evolutionary explanation.
Although mice are nocturnal, many hours of darkness still signal winter, and there is typically less food.
This may be why the mice eat most of their food at one sitting and then fast the rest of the time, even though they are more active. But can these observations be relevant for people?
Lewin Small says that separating the factors affecting people’s seasonal changes in metabolism is extremely difficult.
In winter, people’s photoperiods are no longer solely based on daylight. Most of us experience artificial light at least 16 hours a day.
In addition, culture has a great influence. For example, many people eat more than usual during holiday celebrations in December.
“However, our experiments show that changing the photoperiod even affects the metabolism of animals that do not hibernate. This may affect people, but it definitely affects the animals we use in laboratories to learn more about metabolism, including human metabolism,” says Lewin Small.
He elaborates that the researchers will continue to investigate the evolutionary background for the observations they made in the study.
“Why does food intake differ between winter and summer, and how does this expand knowledge about how the photoperiod affects human metabolism? These are some of the questions we would like to answer,” he concludes.
