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Depression health centre

Seasonal affective disorder may be less common than we think

As summer nears its end, a timely study has looked at the links between changes in the seasons and the weather, and how likely people are to have symptoms of depression.

BMJ Group News

What do we know already?


Seasonal affective disorder (SAD) is a type of depression that causes symptoms that happen in a seasonal pattern, at certain times of the year, usually during the winter.

We know that the weather and the seasons can affect people’s mood. Things like how long the days are and how strong the sun is, as well as the temperature and how much rain, humidity, or wind there is, can all make a difference. But it is thought that, for people with SAD, the weather can affect their bodies in such a way as to make depression more likely. However, much of what we know about this subject comes from studies that rely on people’s own assessments of the causes of their moods, which may not be the most reliable kind of evidence.

The new study included records from 762 people in two areas of the United States. Over a number of years, the participants filled out several questionnaires designed to measure whether they had symptoms of depression or were bothered by other kinds of problems (like having less energy than usual).

The researchers recorded the time of year, and collected information about the average weather conditions in the two weeks before each participant filled out a questionnaire. They then looked to see if there was a link between the seasons, the weather, and how likely people were to have symptoms of depression.

What does the new study say?

When the researchers looked at people’s individual responses to the questionnaires, they found that their symptoms of depression were more severe during the winter months than in the other seasons.

But when the researchers looked at the group as a whole, the link wasn’t very strong. None of the weather patterns they looked at, or the season or time of year, could be used to predict whether people would have symptoms of depression that needed treatment.

Even when the researchers looked only at the participants who had previously had depression, there was still no overall link between the weather and how likely people were to have symptoms of depression.

How reliable is the research?

This study was designed to overcome some of the problems common to other studies in this area, and to make the results more reliable. The researchers used questionnaires that are commonly used to measure depression and precise measures of the time of year and weather. This is more precise than asking people about their general mood or relying on them to provide information about the weather or the season. The study lasted between 19 and 22 years, to build up long-term data about people’s depression symptoms.

However, although this in an improvement on some earlier, less reliable studies, the results are likely to only apply to people in the regions that were studied, or who live in areas with a similar climate. People in other parts of the world may be affected differently, and any link to depression symptoms may be very different.

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