Why does immunity suddenly feel weaker in Summer? Expert reveals the hidden habits that drain your body’s defences

Summer is usually seen as the “healthy” season. The days are longer, sunlight is abundant, and cough-and-cold infections seem less common. Yet many people quietly struggle through these months with fatigue, stomach infections, headaches, dehydration, poor sleep, and a strange sense of exhaustion that refuses to leave.

Doctors say the problem is not always visible at first. The body does not suddenly “lose immunity” overnight. Instead, summer slowly pushes the body into stress. Heat, dehydration, disturbed sleep, irregular eating, and constant movement between extreme temperatures can quietly weaken the body’s natural defence system.

Dr Reshu Agarwal, Consultant Physician at Kailash Hospital, explains that people often ignore how deeply heat affects the body.

She says, “Most people don’t think of summer as a season for falling sick. If anything, it’s supposed to be the ‘healthy’ time of the year. More sunlight, fewer viral outbreaks, fewer colds. That’s the common assumption.”

But the reality looks different inside clinics and hospitals during peak heat months.

“When temperatures are unusually high, your body continuously attempts to keep its temperature down. There is excessive sweating, depletion of bodily fluids, and continuous effort to achieve equilibrium,” she explains.

This constant effort creates physical stress. Even mild dehydration can reduce energy levels, affect concentration, disturb digestion, and increase fatigue. Over time, the immune system also becomes less efficient because the body is busy trying to maintain balance.

A study published in Scientific Reports found that rising heat stress in India is increasing physiological strain and affecting human health in significant ways.

The Government of India’s heatwave advisory also warns that extreme heat can cause “physiological strain” and serious illness if the body is not properly protected.

Summer changes eating habits more than people notice.

Heavy meals feel uncomfortable. Water intake becomes irregular. Street food, sugary drinks, packaged juices, and chilled snacks become more tempting. The result is often acidity, indigestion, bloating, or stomach infections.

Dr Agarwal says, “What most people fail to realize is the connection between gut health and immune function. The inability of the body to digest food well may also affect immunity levels.”

Scientists have long linked gut health with immune strength because a large part of the immune system is connected to the digestive tract. When digestion becomes weak, the body’s defence mechanisms also suffer.

Hot weather also increases the risk of food contamination. Food spoils faster in high temperatures, especially when stored poorly or consumed outside.

Long gaps without water make things worse. Many people wait until they feel thirsty before drinking fluids, but by then the body may already be mildly dehydrated.

​Research from India has shown that prolonged heat exposure and dehydration increase physiological stress significantly, especially among outdoor workers.

One of the most overlooked summer habits is the constant switching between extreme temperatures.

Scorching outdoor heat followed by freezing air-conditioned spaces forces the body to keep adjusting repeatedly throughout the day.

Dr Agarwal explains, “For some, however, this continuous fluctuation can result in irritation of the throat or mild congestion. Though it may not be an actual sickness, it can persist for some period of time.”

Air-conditioned rooms are also dry environments. They reduce moisture in the nose and throat, which normally act as protective barriers against dust, allergens, and germs.

Children and older adults are often more sensitive to these changes. That is why throat irritation, dry cough, mild respiratory discomfort, and headaches become surprisingly common during summer.

Summer nights are becoming hotter in many parts of India. That matters more than people think.

Poor sleep directly affects immune health. When sleep becomes irregular, the body gets less time to repair itself. Hormonal balance changes, stress levels rise, and recovery slows down.

Many people also sleep late during summer because of discomfort, power cuts, screen time, or heat-related restlessness.

Dr Agarwal points out that immunity drops “quietly” through “the combination of heat, dehydration, improper sleeping patterns and small alterations in daily routines.”

The impact may not look dramatic initially. But repeated stress on the body slowly builds up.

There is no magic drink or supplement that can completely “boost” immunity in summer. Doctors say basic habits still work best.

Dr Agarwal advises, “Keeping it basic usually works best. Drink water throughout the day. Eat simple food more often than outside or packaged options. And if you feel tired, it’s better to slow down than keep pushing.”

​
Drink water consistently, not all at once
Include fruits with high water content like watermelon, cucumber, and muskmelon
Avoid excessive sugary drinks and alcohol
Eat freshly cooked meals whenever possible
Maintain proper sleep timing
Limit sudden temperature shifts between outdoors and AC rooms
Pay attention to unusual fatigue, dizziness, or repeated stomach issues

India’s Ministry of Home Affairs also recommends avoiding direct afternoon heat exposure, staying hydrated, and avoiding stale or very heavy foods during heatwaves.

Summer may not always look dangerous on the surface. But the body often sends small warning signs before bigger problems appear. Listening to them early can make all the difference.

This article includes expert inputs shared with TOI Health by:

Dr Reshu Agarwal, Consultant Physician, Kailash Hospital, Noida.

Inputs were used to explain how extreme heat, dehydration, poor sleep, irregular eating habits, and sudden temperature changes during summer can quietly weaken immunity, and what simple daily habits can help protect the body during the hottest months of the year.