Children who watch television late at night could be “more likely to develop depression”, The Daily Telegraph has claimed. The research behind this report has been covered by several other newspapers, which say that streetlights could also be responsible.
This research housed mice for several weeks in a room that was lit 24 hours a day, testing measures thought to indicate depression and distress. These mice showed more depressive symptoms than similar mice exposed to a normal cycle of light and dark. The researchers believe that the findings could apply to humans, as they used the same methods that pharmaceutical companies do in preliminary testing of anti-depressive and anti-anxiety medication.
This was animal research, so applying its findings to humans should be treated with caution due to the numerous major differences between species. Furthermore, the extreme lighting regime tested on mice does not mirror real life in humans.
This study did not test the effects of streetlights or television on human mood, so any conclusions about their effects should be considered as speculation.
Where did the story come from?
This research was carried out by Dr Laura Fonken and colleagues from the Departments of Psychology and Neuroscience at Ohio State University. The study was supported by grants from the National Science Foundation and published in the peer-reviewed journal Behavioural Brain Research.
What kind of scientific study was this?
In this animal study the researchers wanted to test whether constant light conditions produce ‘affective responses’ (changes in mood). They also wanted to see whether these behaviour changes would be the result of differences in concentrations of glucocorticoid, a steroid hormone released by stress.
The researchers took 24 eight-week-old mice and allowed them to drink and feed freely. After a week of getting used to their cages, they were randomly assigned to either the control group or experimental treatment group. The 12 mice assigned to the control group were maintained under a cycle of 16 hours of light followed by eight hours of dark, while the experimental group was maintained in constant light for the remainder of the study.
After three weeks under the different lighting conditions the mice underwent several behavioural tests to measure responses believed by the researchers to be similar to human anxiety and depression. These tests included:
- An open field test, in which total movement was tracked for 30 minutes and analysed for the percentage of specific movements, such as rearing up and the tendency to remain in the centre of the test chamber. Both of these are thought to represent low anxiety responses.
- An elevated maze test in which mice navigated a maze one metre above the floor. The time spent before exploring an open arm of the maze is linked to anxiety.
- Monitoring sucrose consumption, as it is a measure of a mouse’s contentment level.
- In the Porsolt forced swim test, the length of time a mouse spent floating stationary was measured. This length of time is thought to represent a depressive-like response.
Following the testing, the mice were killed humanely and their adrenal glands, spleens, testes and fat pads were collected and weighed. Blood samples were collected before the experimental light condition, two weeks after, and at death.
What were the results of the study?
The researchers say that:
- Mice exposed to light for three weeks had increased depressive-like behavioural responses on the tests.
- The mice exposed to continuous light were evaluated as displaying reduced anxiety in the open field and elevated maze tests.
- Glucocorticoid hormone concentrations were reduced in the continuous light group, suggesting that the behaviour was not the result of elevated corticosterone stress hormone.
What interpretations did the researchers draw from these results?
The researchers say, “Taken together, these data provide evidence that exposure to unnatural
lighting can induce significant changes in affect (mood), increasing depressive-like and decreasing anxiety-like responses [in mice].”
They add that the present study has important implications because it indicates that night-time light may lead to depressive-like disorders.
What does the NHS Knowledge Service make of this study?
This study has shown behavioural changes in mice that have been exposed to continuous light compared to other mice that experienced a normal light/dark cycle. The measures used are fairly standard tests for this type of research, and so it is important for researchers to know if the amount of the animals’ light exposure might be an influencing factor in other research they perform, for example in studies of anti-depressant medication.
As with all animal research, extrapolation of any findings to humans needs to be treated with caution due to major differences between species. Furthermore, the extreme lighting regime tested on mice (constant exposure to light for weeks at a time) does not mirror any realistic situation in human life or in nature outside of the Arctic Circle.
While newspapers have taken these results to mean that street lighting and television can cause depression, it raises the question, why would people not simply close their curtains or switch off when light becomes a disturbance?
The researchers in this paper briefly mention that depressive behaviours in humans “may have evolved under a similar seasonal context as that of rodents”, and therefore that humans may still be susceptible to changes in environmental lighting. Again, this seems a little far-fetched, and is a claim that this research cannot directly support.
The researchers also say that, “Unnatural light cycles to which humans are now exposed, and the irregular sleep patterns evoked by light at night, may interfere with typical responses to the annual cycle of changing day lengths.” If this were the case then it would be far better to test this in humans. Light exposure is not harmful, so there is no obvious reason why these theories could not be tested directly on humans.
Analysis by Bazian
Edited by NHS Website
Links to the science
Daily Mail, October 23 2009
Daily Express, October 23 2009
The Daily Telegraph, October 23 2009