Though the summer has ended, the threat of excessive heat will return, and studies indicate it will get worse. Heat is certainly uncomfortable, and it can make you feel exhausted and irritable, especially when it fails to relent at night.
More than just a subjective feeling, heat stress can have a serious impact on one’s health. Excessive heat can increase the risk of major cardiovascular events like stroke and heart attack. It can also contribute to inflammation, neuroinflammation, and sleep problems. Taken together, these factors can compromise individuals’ physical as well as mental health, especially for those with an underlying condition.
This post explores the effects that heat can have on one’s wellness and is meant to serve as a reminder that your physical health and your mental health are deeply intertwined. It all begins with something that everyone has experienced: stress.
Heat as a Form of Stress
Stress is not just a subjective feeling of irritation or a sense of being overwhelmed; it has a measurable impact on blood pressure, heart rate, respiration, and other functions managed by the autonomic nervous system (ANS). The ANS is divided into two branches, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS is activated by stress or perceived dangers (resulting in fight-or-flight responses), while the PNS is activated when the stressor or perceived danger has passed (resulting in rest-and-digest responses). As a simple analogy, the SNS is like the gas pedal in a car, whereas the PNS is like the brake.
Activation of the SNS begins in a specific part of the brain known as the amygdala, which triggers a cascade of reactions via the hypothalamic-pituitary-adrenal (HPA) axis and the release of epinephrine (adrenaline), norepinephrine, and cortisol. These hormones circulate throughout the body and give us the extra boost in performance we need to fight off a predator or flee to safety.
Unfortunately, stressors like heat can also activate the SNS and keep it engaged over long periods of time. A study published in 2025 by Fricke and colleagues found that even relatively mild heat exposure can lead to increased activity in the HPA axis and SNS. Specifically, they measured mean arterial pressure and heart rate, as well as concentrations of salivary cortisol and α-amylase to measure SNS arousal. They observed a stress response even at 28°C (82.4°F).
As temperature and humidity increase, it only puts more stress on the body, particularly among vulnerable populations. For context, the limit of human survivability is believed to be six hours of exposure to wet-bulb temperatures of 95°F (i.e., 100 percent humidity with an air temperature of 95°F).
The Effects of Heat Stress
Heat stress can be either acute or chronic. Acute cases of extreme heat exposure can lead to heat stroke, which is when a person’s core body temperature rises to 104°F or higher, and can result in organ failure. Common symptoms include flushing, extreme sweating, nausea, confusion, headache, agitation, rapid breathing, elevated heart rate, seizure, and coma.
As explored below, chronic heat stress can also be detrimental to health.
Cardiovascular Stress
Nonemergency heat stress can be taxing on the cardiovascular system, as the Fricke study found. That stress becomes dangerous when compounded by underlying cardiovascular conditions, but even individuals in relatively good health can be at risk of ischemic stroke and heart attack because the heat also changes blood composition, making it more predisposed to clotting—as Keatinge and colleagues astutely observed approximately 40 years ago. More recent research has corroborated the general findings of Keatinge. For example, a 2023 meta-analysis found that excessive heat is associated with a 10-percent increase in risk of stroke morbidity and a 9 percent increase in risk for stroke mortality.
Inflammation
When the SNS remains on alert for prolonged periods of time, it can activate immune pathways, leading to the release of pro-inflammatory molecules. These molecules are designed to protect against nonspecific threats to the body. When these molecules remain in circulation over prolonged periods of time, they can damage healthy tissue and increase the likelihood of inflammation and neuroinflammation. As I’ve discussed in previous blog posts (see here and here), the clinical correlate of neuroinflammation may be psychiatric symptoms like anxiety or depression, or cognitive symptoms like brain fog.
Sleep
Excessive heat makes it difficult to fall asleep and stay asleep because sleep requires a drop in core body temperature. When it’s too hot, people tend to move around more throughout the night, their heart rate remains elevated, they may wake up repeatedly, and they may feel as though their sleep quality is poor. This is not just a subjective feeling. REM and deep sleep stages both suffer during the heat. The body will cut short both stages of sleep if it is too hot, which can contribute to grogginess while also compromising memory consolidation, emotional health, and brain health, as the brain’s cleaning crew works most vigorously during deep sleep.
For patients with underlying psychiatric disorders, including posttraumatic stress disorder, mood disorders, or anxiety disorders, disruptions in their normal sleep schedule can exacerbate symptoms.
How to Keep Your Cool
Beyond using air conditioning, shade, or fans to cool down, it’s important to stay hydrated and to focus on sleep health when temperatures climb. Sleeping in a cool room with breathable bedding can help you get to sleep and stay asleep. Meanwhile, a warm bath or shower well before bedtime can also help the body shed heat and prime the body for sleep. Keeping your cool can also mean finding ways to de-stress through exercise, yoga, or meditation. Meanwhile, eating a diet that limits processed foods and is predominantly plant-based but contains some lean proteins like seafood, eggs, and poultry can help reduce inflammation and cardiovascular stress.
It’s important to remember that the effects of stress on the body are cumulative. Regardless of the source—whether it’s poor sleep, excessive heat, or a bad diet—stressors can negatively impact physical health and mental health. Keeping cool during the summer is not just a matter of comfort; it’s a matter of health.
