What caught Aniruddh Naidu, a communication professional who had recently moved to Delhi from Bengaluru, off-guard was not the culture but the weather. It was a temperature shock of sorts when he really `felt’ the mercury hit over 40°C – a first time for him.
“An unquenchable thirst led to a negative frame of mind, increased irritability and an uncomfortable feeling of being bloated at all times.” The heat also killed his resolve to leave home during the day and made him restless and isolated.
His is just one of the many stories of high temperatures knocking us off guard. Even as scientists debate the exact nature of the relationship, research indicates that there is a direct corelation between rising temperatures and adverse mental health conditions.
The one-degree damage
The journal Environment International in 2021 published a systematic review and meta-analysis of 53 peer-reviewed epidemiological studies on heat exposure and mental health outcomes, all published between January 1990 and November 2020. The review led by Jingwen Liu showed that a 1°C rise in temperature was associated with an increase in incidences of common mental disorders like anxiety and depression as well as severe conditions like bipolar mood disorders and schizophrenia.
The findings also suggested that populations living in tropical and subtropical climate zones and people above 65 years of age were more vulnerable.
Behaviour & temperature
A 2018 review by author Mare Lõhmus from the International Journal of Environmental Research and Public Health provides examples of possible biological mechanisms that could partly explain the heatwave-related worsening of mental disease morbidity. It also reviews the complicated central processes involved in maintaining a stable body temperature in hot environments.
However, there is significantly large evidence that suggests an indirect relationship between the two: that higher temperatures lead to behavioural modifications among the population and result in adverse mental health effects.
Numerous studies have shown that behavioural changes often occur in populations because of elevated ambient temperatures. The American Psychiatric Association reports that extreme heat can contribute to increased aggression, incidences of domestic violence and increased use of alcohol or other addictive substances to cope with the resultant stress.
Another research led by JT Mullens and published in the Journal of Health Economics characterised the link between ambient temperatures and a broad set of mental health outcomes. This 2019 study found that visits to the emergency department for mental illness, suicides, and self-reported days of poor mental health also increased because of higher temperatures.
Interestingly, the researchers also found that incidence of mental health issues dropped with colder temperatures. “Specifically, cold temperatures reduce negative mental health outcomes while hot temperatures increase them,” the researchers said.
The body is adapting
Dr Alok Bajpai, a consultant psychiatrist at IIT Kanpur and Regency Hospital, agrees with this hypothesis. “The brain and body are a homeostatic system. That means it [the body] is constantly striving to maintain certain parameters within specific ranges – that is why you sweat when it is hot and your skin wrinkles when it is cold. These are ways in which the body tries to adapt to external temperatures and maintain the balance needed to protect our internal organs.”
However, these adaptations come at a price, he says. “We have evolved to handle short-term fluctuations to the status quo and can adapt very efficiently. But when these changes are prolonged, for instance with a region experiencing unprecedented heatwaves that the local population is not accustomed to, then the adaptations are often accompanied by side effects that affect our behaviour and can cause adverse health outcomes, including mental health.”
Research shows that the most significant side effect of our bodies’ attempts to adjust to the heat and the primary cause of adverse mental health outcomes is sleep disruption.
The clinical evidence from the 2018 study by Mare Lõhmus suggests that sleep and emotion interact and that nearly all psychiatric and neurological disorders are associated with sleeping problems.
A 2022 study led by author Kelton Minor and published in the journal One Earth found that warmer temperatures reduce sleep globally and amplify the risk of insufficient sleep. Using wristbands with internal accelerometers to measure sleep duration and sleep timing in over 47,000 adults across 68 countries for an average of six months, the study found that the likelihood of getting less than seven hours of sleep increased by 3.5 percent: that is, if minimum outside night-time temperatures exceeded 77°F (25°C) compared with the baseline temperature of 41 to 50°F (5 to 10°C).
Researchers predict that people around the world are likely to lose 50 to 58 hours of sleep a year by 2099 due to global warming.
The study also found that people adapted themselves to colder climates better than to warmer ones, suggesting that already warm geographies that are being impacted by climate change will increasingly disrupt their population’s sleep patterns as temperatures increase over time.