To better treat and prevent depression, we need to understand more about the brains and bodies in which it occurs.

Curiously, a handful of studies have identified links between depressive symptoms and body temperature, yet their small sample sizes have left too much room for doubt.

In a more recent study published in February, researchers led by a team from the University of California San Francisco (UCSF) analyzed data from 20,880 individuals collected over seven months, confirming that those with depression tend to have higher body temperatures.

As thorough as the study is – involving participants from 106 countries – it's not enough to show that a higher body temperature is causing depression, or indeed that depression is leading to the warming up of the body.

However, it does suggest that there's a connection here worth investigating. If something as simple as keeping cool could help tackle the symptoms of depression, then that has the potential to help millions of people around the world.

"To our knowledge, this is the largest study to date to examine the association between body temperature – assessed using both self-report methods and wearable sensors – and depressive symptoms in a geographically broad sample," said UCSF psychiatrist Ashley Mason when the results were published.

There could be a number of reasons for the link, the researchers think. It might be that depression is tied to metabolic processes that generate extra heat perhaps, or tied to cooling biological functions that aren't operating properly.

Or there might be a common shared cause, such as mental stress or inflammation that impacts both body temperature and depressive symptoms separately.

That's something future studies could look into. For now, we know that depression is a complex and multi-faceted condition, most likely with lots of different triggers, and body temperature could be playing a role.

Previous research has found that hot yoga, hot tubs, and saunas can lessen the symptoms of depression, albeit in small sample groups. It's possible that the self-cooling this triggers, through sweating, is having a mental effect too.

"Ironically, heating people up actually can lead to rebound body temperature lowering that lasts longer than simply cooling people down directly, as through an ice bath," said Mason.

"What if we can track the body temperature of people with depression to time heat-based treatments well?"

The study data showed that as self-reported depression symptoms became more severe, body temperature averages got higher. There was also some association between higher depression scores and lower daily temperature fluctuations, but not to a statistically significant level.

With around 5 percent of people around the world thought to be living with depression, efforts to understand and effectively treat it are now more urgent than ever. Each new discovery brings more hope in tackling the problem.

"Given the climbing rates of depression in the United States, we're excited by the possibilities of a new avenue for treatment," said Mason.

The research has been published in Scientific Reports.