Does dirt make us happier? And could night lights or a TV screen on while you sleep cause depression?

A review of research from Emory Univeristy School of Medicine, Atlanta, USA, the University of Colorado and University College London points towards investigating the loss of beneficial micro-organisms as a possible cause of depression.

The authors have used information from laboratory and human studies to explore the idea that there may be a relationship between major depressive disorder and exposure to some types of bacteria. People with major depression disorders can show extreme immune responses (including inflammation) to stress, and inflammation can trigger symptoms of depression. The authors put forward the hypothesis that improvements to hygiene, although reducing risk of infectious diseases, have also disrupted our evolutionary relationships with certain micro-organisms that may have had a positive effect on our physical and mental health.

The micro-organisms were those found in soil, food and faeces and it is human contact with these that has declined in industrialised countries. The authors suggest that reintroduction of these ‘old friends’ (beneficial bacteria) to people affected by depression may reduce their symptoms.

The report was not a conclusive finding of any relationship that might exist between bacteria and mental health: however it was drawing on previous studies and opening a new line of investigation.

Source: NHS news article and the Archives of General Psychiatry


On a similar theme, a study has found that having a dim light on throughout the night can prompt changes in the brain that may lead to mood disorders including depression.

Tracy Bedrosian, a doctoral student in neuroscience at Ohio State University and her colleagues exposed one group of hamsters to 16 hours of daylight and eight hours of complete darkness each day. Another group were exposed to 16 hours of daylight followed by eight hours of dim light, similar to the intensity of the light emitted by a television screen.

After eight weeks of this treatment, the rats were tested for depressive behaviour, for instance whether they engaged in activities that they previously enjoyed, such as drinking sugar water.

The rats who experienced the dim light for eight hours seemed to have lost their enjoyment of drinking sugar water, and this inability to experience pleasure is known as anhedonia, one of the major symptoms of depression.

The changes in the rats behaviour are associated with changes in the area of the brain called the hippocampus, and the rats exposed to the light at night had fewer dendritic spines on the surface cells of this region. Dendritic spines are hair like protrusions that the brain cells use to communicate with each other. Brain changes may arise from fluctuations in the production of melatonin, a hormone that signals to the brain that it is nighttime. A light at night reduces the brain’s production of melatonin, and this particular hormone has been shown to have some antidepressant effects.

Bedrosian suggests that if the same changes occur in human, then it might be better to try falling asleep at night without televisions on, and with minimal light pollution.

Source: and Neuroscience 2010

First Published in December 2010

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