Mind Games: Heat Stress and Cognitive Ability

Your Brain On Heat

Mental functions like sustained attention, vigilance, reaction time, and working memory are negatively impacted by exposure to extreme heat. Despite being only about 2% of our body mass, the brain accounts for about 20% of baseline (i.e., resting) metabolic heat production. (For reference, a human brain’s metabolic rate as a fraction of the body’s total is significantly higher than that of other primates (8–10%) and far exceeds the 3–5% of total metabolic heat production in most mammals). Just like an overclocked computer, human cognition is energy intensive, so our brains run hot.

It’s critical for our body to remove the heat generated inside our skull, and evolution has given us physiological tools to do this well most of the time. However, thermoregulating our brain can be difficult when the environment is hot. Sections of our brain operate at slightly higher temperature than our body core, but the two measurements correlate closely. An elevated core temperature means brain temperature is also higher and, according to a Frontiers in Neuroscience publication, brain cells are “exceptionally sensitive to thermal damage”. Once our core temperature strays too far from an ideal 98.6^oF, our ability to think and react begins to suffer.

This temperature vs cognitive function relationship is a classic “inverted U”, with cognitive performance dropping in extreme heat or cold. One the hot side, researchers have linked cognitive performance decreases to core temperature exceeding 101^oF.[1]

A simplified “inverse U” relationship between core temperature and cognitive performance. For cognitive function, some studies find the ideal ambient temperature is around 72^oF.

It’s All In Your Head?

Before we dive into cognitive performance, it’s worth noting that human performance suffers in the heat even before core temperature increases! The perception of heat alone is often enough to affect physical performance.

One example comes from a deception study where subjects performing cycling time trials were tricked into think the environmental condition was cooler, and their body core temperature lower, than they actually were. Compared to testing when the true, higher ambient and core temperatures were known, cyclists performed better when they believed it was cooler- in fact, they performed just as well as when conducting time trials in actual cool conditions!

Perception matters because, when performing work that can potentially overwhelm our body’s cooling mechanisms, we subconsciously anticipate the need to reduce our effort and “play it safe” well before core temperature starts to rise. Compared with the same effort in cool conditions, integrated electromyographic activity (a measure of the electrical activity of muscles) is lower in subjects performing self-paced exercise in the heat even before core temperature begins to rise. Researchers attribute this to an anticipatory response that limits muscle recruitment and power output as a response mechanism to reduce metabolic heat production well before there is the threat of heat stroke. Put another way, we subconsciously throttle back our efforts in hot conditions to protect ourselves from heat injury.

These and similar studies suggest the perception of heat, and not physical effort alone, limits the maximum working effort we can achieve in hot conditions. While people occasionally push beyond their thermoregulatory limits (as noted in the Journal of Special Operations Medicine, high levels of motivation are a leading factor for heat stroke), most of us will rarely, if ever, force our bodies beyond safe metabolic effort.

Higher Level Thinking & Heat

Researchers attempting to quantify the effects of temperature on cognitive performance find mixed results depending on study population, environmental conditions, and cognitive task. This work has led researchers to study interesting situations, such as the world of competitive chess. A 2019 study of competitive chess players found matches during warmer temperature did not appear to correspond with more errors during play. However, chess tournaments are held indoors, and the temperature range across the 14 matches studied varied only slightly (between 72-84^oF). While 84^oF may be a bit warm for an indoor venue, it is unlikely to create heat stress conditions.

On the other hand, studies of student test scores clearly show reduced performance, especially in math (a “higher level” cognitive task), when testing occurs during higher temperatures. A 2017 study found outdoor temperatures between 86-89.6^oF decreased student’s math score by 1.6 percentile points. (“Percentile rank” is relative to one’s peers; a score in the 90^th percentile means you are the top 10% compared to your peers). In this study, researchers found decreased cognitive performance occurs even given “moderate to high levels of air-conditioning”. Overall environmental living conditions, not just the immediate testing environment, appeared to matter. A separate study in New York City public schools found hot days reduce academic performance up to 15% despite air conditioning available in most schools.

Poor test performance due to heat is not limited to children. Similar findings are published in a studycomparing college students with air conditioned dorms to students without A/C (the study was in Massachusetts, a state cool enough that air conditioning, though common, is not universal). During heatwaves, students without air conditioning “experienced significant decrements” between 6-10% lower on tests measuring cognitive reaction speed, inhibitory control, and working memory. Since the tests were conducted in the student’s room, the researchers attributed the decreased cognitive function to their overnight thermal environment.

Occupational and Military Cognition

 Those examples are a bit academic (pun intended) but provide evidence that extreme heat negatively affects our ability for higher-level thought. Research documents this cognitive decline in working adult and military populations as well.

Soldiers in the Australian Defence Forces often operate in hot, humid conditions. To evaluate the effect of hot conditions on Australian soldier cognitive performance, a study used Solid State Probe Topography, a brain imaging technique, to measure changes in brain activity. The subjects were also given tests measuring working memory, attention, concentration, and information processing speed after their core temperature was elevated to mild hyperthermia (to around 101.6^oF in one iteration) by exercising in hot (95^oF) conditions. Due to the small number of subjects (11 total), findings were mixed, but the researchers concluded visual inspection time and working memory were both negatively affected. Changes in brain electrical activity were also observed during heat stress.

A larger study of forty Serbian soldiers conducted at the Military Medical Academy in Belgrade found similar results. This test protocol pushed core temperature higher than the Australian study (up to ~103^oF) and lasted longer, placing the Serbian study cohort under greater thermal stress. Unsurprisingly, the study group performed much worse on tests measuring cognitive performance during complex tasks. The heat stressed cohort also had slower reaction times.  

Studies from other nations have similar findings. A study of 100 Indian soldiers in desert conditions found “significant decline” in cognitive performance in a hot climate, with tests requiring sustained attention, concertation, and memory especially affected.

Reduced cognitive performance isn’t limited to soldiers on the ground. The cockpit of combat aircraft can get brutally hot, especially on the tarmac and at low altitude. One older U.S. study found crews flying the F-4 jet (used by the Navy and Air Force until 1996) had more target errors during low level summer flights in hot conditions. A similar Chinese study simulating cockpit conditions for pilots found perception and reaction flexibility decreased, and error rates rose, during high temperature testing in a simulated fighter cockpit.

One Israeli military study of 500 helicopter incidents found the risk of accident was 500 times (!) more likely when ambient temperatures rose above 95^oF compared to cooler (77-84^oF) conditions. With findings like these, it’s unsurprising that the U.S. Navy tested ice vests during high-heat flights over the Persian Gulf in the early 1990’s.

Heat & Aggression

One of the more surprising ways heat stress affects cognition is the effect it has on human emotion and self-regulation.

Criminal activity increases in hot weather, and researcher consistently find heat increases aggression.  Relative to cooler weather, one study found crime in Los Angeles increases almost 2% on days when the maximum temperature exceeds 90^oF. A separate meta-analysis of 83 studies from around the world found, on average, a 18^oF increase in temperature corresponds with a 9% increase in violent crime.

Lesser forms of aggression also increase during heatwaves. A study in Taiwan found road accidents up 8.8% on days when temperature exceeds 86^oF because of an increase in “risk behaviors” by male drivers during “weekday peak hours” (i.e., rush hour traffic).

Heat doesn’t just affect overt aggression and crime; it also operates in more subtle ways. As mentioned earlier, heat affects higher level cognitive ability. One might assume Federal immigration judges- experienced, professional decision makers, working in air-conditioned rooms- are unaffected by high outdoor temperatures. Yet a 2019 study finds they are. The study, which analyzed over 200,000 court files, suggests a 10^oF increase in outdoor temperature corresponds with a 1% decrease in favorable outcomes for applicants.

This result is fascinating because it links hot temperature with decisions made by a professional, presumably unbiased population of federal judges operating within a legal framework at court locations across the country. This is corroborated in other studies finding judges dismiss fewer cases, issue larger fines, and impose longer prison sentences when ruling on hot days.

Conclusion

When temperatures spike, we experience reduced mental performance, especially with higher level mental tasks. We have slowed reaction times and a harder time learning. Our ability to recall information suffers, we experience elevated levels of aggression, and have a harder time with impulse control.

As temperatures across the globe continue to rise, we need adaptations to keep cool and stay sharp. Qore Performance products offer adaptations to keep a cool head in hot conditions.