Not just hot days, hot nights are the real problem: Why heatwave health impact and risk increases after sunset

The month of May has not started yet but many parts of India are reeling under an intense heatwave. The temperature in Akola in Maharashtra has almost touched 47 degrees. According to India Meteorological Department or IMD, the heatwave conditions are likely to continue over many parts of the country. Health experts have cautioned heat-related illnesses. With heatwave conditions expected to persist, authorities have advised people to avoid direct sunlight during peak hours, stay hydrated, and keep ORS readily available to prevent serious health complications.

What often gets less attention though is what happens after sunset. Hot nights also pose a serious risk to human health because they prevent the body from cooling down and recovering after an extremely hot day. Continuous exposure to high temperatures, even after sunset, can intensify stress on the body.

Why the body struggles on hot nights

The average human body temperature is around 36.9 degrees Celsius. When the external temperature rises significantly above this level, the body begins absorbing heat from the surroundings. To counter this, it activates cooling mechanisms such as sweating and increased respiration to maintain internal balance. However, prolonged exposure to high temperatures can overwhelm these natural processes.


Excessive sweating leads to significant loss of water, glucose, and essential salts, increasing the risk of dehydration. If not addressed promptly, this can lead to symptoms such as muscle cramps, dizziness, fatigue, and in severe cases, heatstroke and loss of consciousness.

At night, the body is supposed to cool down and reset. But when temperatures stay high, that recovery phase gets disturbed. Sleep quality drops, and the body remains under stress for longer hours than expected. Over time, this can quietly affect multiple systems in the body.

What research is now showing

According to findings referenced by the National Institutes of Health (NIH), high ambient temperatures are linked with a sharp rise in mortality risk. While daytime heat has long been studied, newer research is showing that night-time heat may carry its own independent risks.

A long-term study based on data from London between 1993 and 2015 analysed how night temperatures affect mortality. The results showed that the impact of night-time heat remained even after adjusting for daytime temperatures. The risk was especially high for stroke, with relative risk significantly increasing at higher temperature levels.

The study also found that hot days followed by hot nights were more dangerous than hot days followed by cooler nights. Night-time heat exposure showed stronger links with chronic ischemic conditions and stroke, and even affected people below the age of 65.

Researchers concluded that night-time temperatures make an additional and important contribution to heat-related deaths. The effect was strongest when warm nights came after already hot days, which is now being used as a basis for heat warning systems in some countries.

Heat, sleep and rising health risks

There is also a growing understanding of how hot nights interfere with sleep. The body needs a drop in core temperature to fall asleep properly. When that does not happen, sleep becomes lighter and shorter.

Poor sleep due to heat is not just about feeling tired the next day. It can impact the immune system, increase inflammation, and raise the risk of cardiovascular problems. Over time, repeated nights of poor sleep can add to long-term health issues, including mental and metabolic disorders.

Urban areas are particularly at risk due to what experts call the “urban heat island” effect. Buildings, roads and lack of green cover trap heat, keeping cities warmer even at night. This means people living in cities may face higher exposure to night-time heat compared to rural areas.

Public health concern

The World Health Organization has already flagged heat as a major environmental health risk. Heat stress is known to worsen conditions like heart disease, diabetes, respiratory issues and mental health problems. It is also one of the leading causes of weather-related deaths globally.

Data suggests that heat-related deaths have been rising, especially among older adults. Between 2000–2004 and 2017–2021, mortality linked to heat among people over 65 increased sharply. Each year, hundreds of thousands of deaths worldwide are associated with extreme heat, with a large share coming from Asia.

What is worrying experts now is that climate change is not only making days hotter, but nights as well. In many regions, night-time temperatures are rising faster than daytime temperatures. This means the body is getting less and less time to recover.

In simple terms, heat is no longer just a daytime problem. When nights stop bringing relief, the health risks go up quietly but steadily. That is why experts are now saying that managing heat exposure should include both day and night, not just what happens under the sun.