Following on from the previous posts, I’ve taken a look at the extant literature on the relationhip between mood and latitude. Doing a search of pubmed with the search term “mood and latitude” retrieved 500 results. I have included the results that I found of interest below. In the previous post, I have outlined a model of the relationship between one aspect of mood and latitude and this is relevant in terms of reviewing the studies.
The researchers in this 1991 New Zealand study across 9 degrees of latitude found a peak for admissions for mania during the Spring and Summer and found variation according to latitude.
This 2017 Japanese study found a relationship between latitude and hyperthymic temperament. This 2015 study by the same group found evidence that temperature mediates the above relationship. This 2014 study by the same group found evidence that daylight maintained hyperthymic temperament in a dose dependent way. The same group found an effect of lower latitude on inducing hyperthymic temperament in this 2012 study.
The researchers in this study postulate the secondary hypothesis mentioned in the previous post in a simplified form, namely that latitude modifies the course of Bipolar Disorder. They provide evidence of an increased prevalence of manic episodes compared to depressive episodes in 439 subjects in India. The results are intriguing but as per the previous post, it is likely that temperature is a confounding factor.
This 1995 Italian study looked at 543 self-reported surveys of seasonality in mood and did not find a relationship with latitude. The researchers in this 2009 Finnish study did not find a relationship between Beck Depression Inventory scores or Global Seasonality Score with latitude. The researchers in this 2003 Canadian study did not find an effect of latitude on seasonal variation in Bipolar Disorder.
There are a number of polar studies. This 1995 study looked at 119 people who stayed in Antarctica over the winter and found an increase in global depressive symptoms. The researchers suggested that social isolation played a role. The researchers in this study looked at 91 people in Antarctic research stations. They found evidence that sleep was affected by daylight hours and this was linked to a delayed effect on mood. This 2014 study looked at 162 adults in Tromsø, Norway (at an Arctic latitude but also a coastal city warmed by the Gulf stream). The researchers found that Depressive symptoms modified the adaptation of the sleep cycle to the daylight conditions. Tromsø, Norway was compared with Accra, Ghana in this 2012 study. Daylight hours were linked to mood in the Norwegian city.
In this 2010 Plos One study, the researchers looked at Google searches across a large number of geographical locations. They found that higher latitudes were associated with a larger seasonal variation in searches. They found that seasonal changes in temperature were strongly correlated with mood-related search terms.
There are studies investigating secondary factors (a few of which could be potential confounders in studies investigating the primary relationship). Thus these researchers identified a relationship between sleep duration and latitude in Chile. Latitude was found to be associated with Vitamin D deficiency in this study. The authors of this 2012 paper look at the effects of residence in the Arctic or Antarctic, identifying the profound effects of prolonged or absence of daylight at these latitudes. These include cardiovascular and metabolic effects. The researchers in this 2014 GWA study looked at photoperiod adaptations according to latitude and suggest a link with neuropsychiatric conditions. The researchers in this 2007 study commented on the effects of latitude on comorbidity of Seasonal Affective Disorder with Schizophrenia. The authors here, note a latitude hypothesis for Seasonal Affective Disorder. The researchers in this 2000 Norwegian study found that seasonal variation in episodes of violence was modified by latitude.
Reiterating the primary hypothesis that at a higher temperature Bipolar Depression may switch to Mania, there is evidence for and against this hypothesis above. The conditions in the polar regions are atypical not just because of the temperature but also the daylight conditions, being at times considerably extended and at other times absent. In terms of mood, the polar conditions are likely to modify mood in a very different way to the primary hypothesis. In the poles, sleep is modified by the extreme daylight conditions with a possible secondary effect on mood. The primary hypothesis is likely to apply to much higher temperatures, perhaps 30 degrees Celsius and above and might also be dependent on the rate of increase in temperature. There are some interesting results above including the Japanese studies on temperament which may support the hypothesis. However, the hypothesis would benefit from a rigorous assessment, collecting data on all of the relevant variables.
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