Why heat wears us out, and the warning signs to know

How the body copes with heat

The body keeps its core temperature within a narrow range whatever the weather. Korea’s disease agency (KDCA) describes three responses to heat: widening the blood vessels in the skin to send more blood there, lowering basal metabolism to make less heat internally, and sweating to lose heat through evaporation.

None of this is free. The water and salt lost in sweat have to be replaced, and WHO explains that the strain of cooling stresses the heart and kidneys, and that heat can worsen existing conditions such as cardiovascular disease, diabetes, asthma and mental health problems. The sense of being worn out without having done anything comes from this unseen work. See the heart and the kidneys, which manage water and salt.

A little water loss goes a long way

In a 2012 study, researchers at the University of Connecticut compared 25 healthy young women on days when exercise left them mildly dehydrated with days when they stayed well hydrated. Dehydration averaged about 1.36% of body mass. Even at that level, vigour fell, fatigue rose, tasks felt harder, concentration dropped and headaches increased, although most measures of cognitive performance did not change.

It is one small study, and dehydration came from exercise rather than heat, so it can’t be generalised far. Still, it suggests that some of the heavy, foggy feeling of summer may come from fluid that hasn’t been replaced.

Hot nights and sleep

Sleep and temperature regulation are closely linked, so the temperature of the room changes how we sleep. A 2012 review of thermal environments and sleep found that, in real-life conditions with bedding and clothing, heat increased wakefulness and reduced slow-wave and REM sleep. Humidity added to the heat load during sleep.

So the day is spent coping with heat, and the night that should restore you becomes lighter. That helps explain why summer fatigue can build up over several days.

Drinking and eating: the basics

• Drink early and often, not only when thirsty. For activity on hot days, KDCA suggests about 500 ml beforehand and a cup every 20 minutes during.
• Don’t skip meals. Replacing a meal with a cold sweet drink makes it hard to cover both fluid and energy.
• The studies and public guidance this article draws on offer no evidence that a particular restorative dish or “summer superfood” relieves heat fatigue. Eating familiar, light, balanced meals as usual is the baseline.
• Rest in shade or air conditioning during the hottest hours, and wear light-coloured, breathable clothing (KDCA).

When it’s more than tiredness

WHO calls heat stress a leading cause of weather-related death and, based on studies of 2000 to 2019, estimates about 489,000 heat-related deaths a year. KDCA lists people aged 65 and over, children under five, pregnant women, people with chronic illness and older people living alone as most vulnerable.

Heat exhaustion

The CDC (NIOSH) lists headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating and reduced urine output as signs of heat exhaustion. According to KDCA, body temperature is normal or slightly raised and rarely above 38.9 °C. Move to a cool place, rest, sip cool water often, and get checked by a clinician.

Heat stroke

Heat stroke is an emergency. KDCA notes that body temperature can exceed 41 °C and people often lose consciousness suddenly. The skin is often hot and dry, but the CDC adds that some people sweat heavily. With confusion, slurred speech, reduced consciousness or seizures, call emergency services at once, move the person somewhere cool, remove outer clothing, and cool them with cool water or wet cloths on the head, neck, armpits and groin. Give nothing to drink to someone who is not fully conscious, because of the risk of choking.