Hidden Dehydration: Are You Losing Focus in the Heat? By the Time You Feel Thirsty, Dehydration May Already Be Underway

Kana Yamamoto, M.D., Ph.D

Physician, Medical Governance Research Institute

In her regular column “A Little Medical Notebook,” where she discusses everyday topics and news from a medical and health perspective, Dr. Kana Yamamoto, an internist at Navitas Clinic and the Medical Governance Research Institute, examines the issue of “hidden dehydration.”

When people hear the word “dehydration,” many think of older adults or infants. However, active, healthy young people who are confident in their stamina may actually be more vulnerable to hidden dehydration because it is harder for them to notice. It is not uncommon for people to become deficient in body fluids without ever feeling thirsty.

I was one of those people myself. I used to drink very little water. Most of my daily fluid intake came from caffe lattes, and even with meals I would drink only a small amount of tea. Although I knew that coffee and tea have diuretic effects, I tended to think, “I’m not thirsty, so I probably don’t need to drink.”

That mindset changed after I moved to San Diego. Summers here are hot, but the air is very dry. Sweat evaporates quickly, making it difficult to notice how much water your body is losing. I began to feel thirsty much more often than I had in Japan and finally realized that I might not be drinking enough.

Of course, summers in Japan are also extremely hot and humid. However, I spent most of my day indoors in air-conditioned environments, only going outside when traveling from place to place. As a result, I rarely sweated heavily and seldom felt thirsty. Looking back, that “too comfortable” environment may actually have allowed dehydration to develop quietly.

Why Air-Conditioned Rooms Can Be Deceptive

Research has shown that in cool, air-conditioned environments, people may fail to recognize thirst even when they are already dehydrated. A review article published in 2010 reported that, under cooler conditions, it takes longer for people to perceive thirst despite a reduction in body water. In other words, comfort can make the risk harder to detect.

More recently, researchers have also focused on what is sometimes called “hidden dehydration” of the airways. A 2025 study found that spending long periods in low-humidity indoor environments can dry out the lining of the nose and throat, leading to inflammation and reduced protective function. Even in humid countries such as Japan, this can occur in rooms where air conditioning or dehumidifiers are running.

In addition, a 2024 study demonstrated that young adults performing cognitive tasks while mildly dehydrated showed significantly worse performance on tests such as the Go/No-Go task and the Stroop test. Reaction times slowed and accuracy declined. These findings suggest that even mild dehydration can negatively affect concentration, judgment, and thinking ability—an issue that students and working adults cannot afford to ignore.

How “Unnoticed Dehydration” Progresses

There are several common situations in daily life where dehydration tends to develop.

People who spend long hours in air-conditioned offices or homes often feel less aware of the outdoor heat and may therefore pay less attention to hydration. Frequent consumption of coffee or energy drinks can worsen the problem because their diuretic effects promote fluid loss, allowing dehydration to progress unnoticed.

People who exercise regularly are not immune either. Many assume that drinking water after sweating is enough. In reality, some replace only water while failing to replenish electrolytes such as sodium. This is particularly important for those who frequently engage in activities that produce heavy sweating, including running, sauna use, and hot yoga. Replacing both fluids and minerals is essential.

Rethinking the “Drink When You’re Thirsty” Rule

Taken together, these findings suggest that the traditional advice to “drink when you feel thirsty” may need reconsideration. By the time thirst appears, the body has often already lost a significant amount of water.

In Japan especially, where many people spend long hours in cool, dry indoor environments, consciously drinking fluids at regular intervals is important. Carrying a water bottle and aiming to drink about one glass of water every hour can help. If you frequently consume caffeinated beverages, simply replacing one of those drinks with water can reduce the risk of dehydration.

These small habits can make a meaningful difference in maintaining concentration and overall well-being. Regardless of age, what this summer may require from all of us is greater awareness of hidden dehydration and a few simple changes to our daily routines.

This article was originally published in Japanese in AERA DIGITAL on July 23, 2025.