Heat wave alert: AIIMS doc explains how Delhi’s warm nights may not let your body recover

Doctors warn that while people often focus on daytime heatwaves, night-time heat may be equally dangerous because it silently prevents the body from cooling and resetting itself.

Human beings are biologically designed to cool down at night. “During prolonged heat waves, survival is not only about avoiding the afternoon sun, it is also about ensuring the body can cool and recover overnight. During sleep, the body’s core temperature drops, helping trigger deep, restorative sleep and allowing organs to recover from daytime stress. But when temperatures remain excessively high after dark, that natural cooling mechanism gets disrupted,” says Dr Neeraj Nischal, professor of medicine, All-India Institute of Medical sciences (AIIMS), Delhi. “We often focus on daytime temperatures but persistently warm nights may be equally important predictors of heat-related complications. People continue to experience heat stress even while resting,” he adds.

How warm nights affect sleep

This creates what doctors describe as a “cumulative heat burden” — a condition in which the body remains under continuous physiological stress for days at a stretch because there is no opportunity to reset between successive hot days.

One of the earliest consequences is disturbed sleep. People often struggle to fall asleep, wake up repeatedly through the night, or experience shallow sleep accompanied by sweating and discomfort. “Sleep deprivation caused by hot nights affects concentration, mood, productivity and the body’s ability to tolerate heat the following day,” Dr Nischal says.

Researchers have increasingly linked elevated night-time temperatures to fatigue, irritability, impaired cognition and worsening mental stress. Doctors say repeated sleep disruption can also weaken immunity and reduce the body’s resilience during prolonged heatwaves.

Watch out for dehydration

Night-time heat silently increases the risk of dehydration. Even while asleep, the body continues sweating in an attempt to cool itself. This leads to continuous loss of fluids and essential electrolytes such as sodium and potassium. “The human body depends on cooler nights to restore thermal balance. When night temperatures remain elevated, the body continues to sweat and lose fluids even during sleep, increasing the risk of dehydration by morning,” Dr Nischal explains.

As dehydration often develops unnoticed overnight, people may wake up already dehydrated the next morning and become more vulnerable to heat exposure during the day. “The result can be weakness, dizziness, muscle spasms, dry mouth, and headaches. If fluids are not replenished, the risk of dehydration increases significantly, especially among the very young and the very old,” says Dr Shailesh Sahay, Senior Director, Internal Medicine, Max Hospital, Gurugram.

“As doctors, we often see heat-related illness being dismissed as simple fatigue until dehydration, electrolyte imbalance or heat stroke suddenly turns life-threatening. Heat illness is not caused by temperature alone; it occurs when the body’s natural cooling system can no longer keep pace with environmental stress,” explains Dr Nischal.

Impact on the heart

The cardiovascular system comes under strain during hot nights. To release excess heat, the body pushes more blood toward the skin surface, forcing the heart to work harder even during sleep. This additional stress can become dangerous for people with hypertension, heart disease, diabetes or kidney disorders. “Your resting heart rate could go up by roughly 10 beats per minute for every 1 ℃ rise in internal body temperature. As blood vessels widen to radiate heat, your BP could drop low. For those with pre-existing heart conditions, the combined stress of elevated temperatures, high humidity and poor sleep can trigger irregular heartbeats, heart failure or heart attacks,” says Dr Sahay.

Dr Nischal warns that some commonly prescribed medicines may further increase vulnerability. “People taking medications such as diuretics for hypertension may face a greater risk because these worsen dehydration. Such individuals may lose fluids overnight and begin the next day with an already reduced water balance.”

The problem of delayed diagnosis

Children and the elderly remain among the most vulnerable groups. Both doctors say children may not recognise or communicate symptoms of heat stress early enough, while older adults often have a diminished ability to regulate body temperature through sweating. “By the time confusion, dizziness or reduced sweating appear, the condition may already be serious,” Dr Nischal says.

Delhi’s urban landscape further worsens the problem. Concrete buildings, asphalt roads, flyovers and densely packed neighbourhoods absorb heat throughout the day and slowly release it overnight — a phenomenon known as the urban heat island effect. Rising humidity, shrinking green cover and poor ventilation in crowded settlements mean that indoor temperatures often remain dangerously high long after sunset without us even realising it. “Public awareness is as important as hospital preparedness during heat waves because early action at home can prevent critical illness,” Dr Nischal says.

How to prepare for night heat stress

Both experts advise maintaining hydration through the evening, ensuring ventilation indoors, wearing loose cotton clothing, avoiding heavy meals late at night and using cooling methods such as fans, cool showers or damp towels before sleep. “Pre-cool your room with blackout curtains. Use a fan to circulate cool air from the AC set to no lower than 24 ℃,” says Dr Sahay.