"Sleeping in a room with too much light has been linked to an increased risk of piling on the pounds," BBC News reports. The news comes from a study that assessed self-reported sleeping habits and body weight measurements in a group of women at a single point in time.
The researchers did find a significant link between light levels in women's rooms at night and their risk of being overweight and obese. However, the study could not provide evidence that light was causing the difference in obesity risk. The authors were aware of this limitation and cautiously described their results as "intriguing".
The researchers speculated that melatonin may play a role in underpinning this link. Melatonin is a hormone whose production is inhibited by exposure to light and is thought to play a role in metabolism.
Sleeping in a darkened room is recommended as it helps promote better and more refreshing sleep patterns.
But this study doesn't provide evidence that simply darkening your room at night will help you lose weight or stop weight gain if you regularly eat more and exercise less than you should.
If you want to lose weight, try the NHS Choices weight loss guide to get started.
The study was carried out by researchers from the University of Oxford and was funded by Breakthrough Breast Cancer and the Institute of Cancer Research.
It was published in the peer-reviewed American Journal of Epidemiology.
The UK media's reporting was accurate and explained plainly that the researchers were not sure if light levels at night were causing obesity or, if so, how this might occur.
This was a cross-sectional analysis of women taking part in the Breakthrough Generations study. This is a long-term ongoing cohort study of women aged 16 years or older living in the UK which aims to identify the causes of breast cancer.
Carrying too much fat (obesity) can damage your health and is known to influence the risk of developing breast cancer, so it was one of the factors assessed in the study.
The researchers also reported that animal studies have shown light exposure causes weight gain, even when energy intake and physical activity (the main influencers on body weight) are kept the same.
They wanted to investigate whether disruption in sleep caused by too much light at night might be contributing to obesity. They say this theory has not yet been investigated adequately in humans.
As this was a cross-sectional analysis, it assessed both the exposure (light at night) and outcome (weight or body fat) at one single point of time. This means that it cannot prove cause and effect as it cannot tell us what came first: whether obesity causes women to sleep in lighter rooms, or if lighter rooms cause women to become obese.
The alternative explanations are that it is simply a spurious link caused by an additional factor (confounder) or that the link is a statistical anomaly that may later be disproved.
The research used questionnaire data from 113,000 women in the Breakthrough Generations study. The women were aged 16 years or older, lived in the United Kingdom and recruited between 2003 and 2012.
The participants were asked about light levels in their room at night. The researchers then looked for links between this and different measures of their weight.
The women were asked to rate the amount of light in their bedrooms at night as:
The two lightest levels were combined into one category because of low numbers in each group.
Their answers were compared with different measures of body weight (BMI) and body fatness (waist to hip ratio, waist circumference, and waist to height ratio).
The group were mainly Caucasian (98.8%). The participants' ages ranged from 16 to 103, with an average age of 47.
The main analysis took into account factors other than light levels that might influence women's obesity risk (confounders).
The fully adjusted analysis took into account:
In the group, 1.3% were underweight, 52.3% were a healthy weight, 28.9% were overweight and 13.7% were obese. A further 3.8% had missing information.
The probability (odds) of women being overweight or obese was progressively lower among those who slept in darker rooms. This link was seen for the middle and darkest levels, as well as BMI and waist to hip ratio.
For example, women sleeping in the darkest rooms were 21% less likely to be obese than those sleeping in the lightest rooms, as measured by BMI (odds ratio [OR] 0.83, 95% confidence interval [CI] 0.79 to 0.88).
No links were found between women being underweight and light levels at night.
The researchers state that, "In this analysis of over 113,000 United Kingdom women, [different measures of weight and body fatness] increased with increasing lightness of the room slept in at night.
"These associations were still present after adjustment for age, socioeconomic status, alcohol consumption, strenuous physical activity, night shift work, having a young child, sleep duration, and current smoking."
However, the study authors are appropriately cautious and distance themselves from saying that light exposure at night could cause obesity, instead describing their results as "intriguing".
Professor Anthony Swerdlow from the Institute of Cancer Research told the BBC that, "There might be other explanations for the association, but the findings are intriguing enough to warrant further scientific investigation."
In terms of the impact on people today, he said: "There is not sufficient evidence to know if making your room darker would make any difference to your weight."
This cross-sectional study found a link between light levels in women's rooms at night and their risk of being overweight and obese. However, it is not able to prove that light caused the difference in obesity risk.
The authors fully acknowledge this and cautiously described their results as "intriguing" and warranting "further scientific investigation".
The main drawback of this study was that it was cross-sectional. This means it cannot prove cause and effect; it can only highlight potential associations.
The study raises the question of how differences in light levels at night might affect obesity levels. When considering whether these results might be showing a real effect, the scientific community will need to think of plausible explanations for the results and carry out further studies to test these.
One theory is that too much light while you are trying to sleep disrupts people's natural biological clock, which has developed over millions of years in response to natural light cycles of dawn and dusk.
However, the association could also run in the other direction. Obese people may be heavier sleepers, so they are more likely to sleep in lighter environments.
There has been increasing interest in potential negative biological reactions to artificial light and disrupted sleep and their impact on biological and mental functioning. No doubt research in this area will continue.
The results may be "intriguing" from a research perspective, but for the average person this study does not provide any convincing evidence that darkening your room at night will help you lose weight or stop weight gain, or, conversely, that sleeping in a lighter room will cause you to gain weight.
For now, the message about how to maintain a healthy weight is simple and unchanged: eat a healthy balanced diet and get plenty of exercise.