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"Close the window, or you'll catch a cold!" You've probably heard this before. Of course, we know it's nonsense – it's viruses and bacteria that make us sick, not cold or drafts. The so-called open-window effect has actually nothing to do with a literal open window but describes a phase after training during which we are more susceptible to illnesses. Today we want to find out what it is all about.

By the way, if you want to know how to optimally support your immune system, check out my article: Immune System 

What is the Open Window Effect?

The Open Window Effect describes a phase after intense physical exertion, during which the immune system is temporarily weakened, making the body more susceptible to infections. This often occurs after intense workouts, such as a long run or heavy strength training, but can also happen due to other stressors like lack of sleep, work pressure, or emotional stress.

During these phases, the production of stress hormones like cortisol increases, which negatively impacts the immune system and weakens the body's defenses. As a result, the body becomes more vulnerable to pathogens, leading to an increased susceptibility to upper respiratory infections. Once the body recovers and stress hormones return to normal levels, the immune system stabilizes, and the risk of infections decreases.

Duration of the Vulnerable Phase

How long and when the Open Window Effect occurs depends on the type and intensity of the training. This effect is particularly noticeable after extreme physical exertion, such as a marathon or heavy strength training.

After training, an "open window" of immune dysfunction begins a few hours post-workout and typically lasts for about 10 hours. During this time, the immune system is at its weakest, increasing the risk of infections, such as respiratory illnesses.

However, it’s difficult to determine exactly how long the "open window" stays open for each individual. As mentioned earlier, this largely depends on the type and intensity of the training, as well as individual factors like overall health and fitness.

In extreme cases, the heightened susceptibility to infections can last anywhere from 3 hours to up to 72 hours.

The vulnerability gradually decreases as the body begins its recovery process and the immune system regains its full functionality.

For example, if you do intense strength training every other day, along with cardio training, your immune system may be more vulnerable to infections over longer periods.

It may seem contradictory at first because we exercise to stay healthy and strengthen the immune system, but immediately after a workout, the body is temporarily more susceptible to infections due to the Open Window Effect, which can reverse the desired effect.

But this very mechanism has long-term positive effects because, during the recovery phase, the body adapts to the stress and further builds its defenses. It becomes more efficient at handling stress, and the immune system overall becomes more resilient. The key here is the balance between exertion and recovery. By allowing enough time for rest, you ensure that the body can make these adaptations optimally, and the immune system is strengthened in a sustainable way.

Avoiding exercise altogether would be the worst thing you could do for your body. Those who move little have a generally higher risk of getting sick and weaken their immune system in the long run.

The key is a balanced training routine that includes moderate strength and cardio training. Equally important is giving the body enough time to recover. Only then can the positive effects of exercise take full effect, and the immune system will be strengthened in the long term.

How the Window is Opened by Other Factors

The window opens especially after intense exercise, but it’s not exclusive to that. Other factors can also weaken the immune system and have similar effects. So, the next time Aunt Olga tells you to dress warmer when it's cold outside to avoid catching the flu, her statement may be incorrect – after all, it’s not the cold, but viruses that make you sick – but she’s not entirely wrong either. Cold does strain the immune system, as the body has to expend more energy to regulate temperature. This makes the body more susceptible to viruses and bacteria.

That doesn’t mean you have to bundle up all the time. And if you enjoy cold water swimming, you can continue doing that. Just be sure to avoid contact with sick people afterward (which is always a good idea in general).

Just like with exercise or cold, the Open Window Effect can also occur due to other things that are bad for health, such as stress, sleep deprivation, poor nutrition, or excessive alcohol consumption. During these phases, the body becomes more susceptible to illness because the immune system isn’t functioning at its best. The difference, however, is that these factors can weaken the immune system and overall health in the long term, potentially leading to chronic diseases over time.

How to Minimize the Risk of Getting Sick After Training

Unfortunately, there’s no miracle pill to protect you from getting sick after intense training. The only thing you can do is avoid making your training plan too intense – sometimes less is more. Sufficient recovery is key to not overburdening the immune system. It's particularly important to remember that you can only get sick if you're exposed to pathogens. So, make sure to follow a few basic rules, especially during the phases when the Window is open, and even more so when flu season starts.

Rule #1: Avoid Contact with Others

This might sound harsh, but it’s the truth. Most people act like pigs. They don’t wash their hands after using the toilet and don’t keep their distance when they’re sick. Instead, they cough around without hesitation, spreading their germs.

During the peak of the COVID-19 pandemic, I hoped that hygiene awareness would rise. Unfortunately, that hope didn’t quite pan out. So, we have no choice but to take responsibility ourselves and protect ourselves from these situations – especially after intense strength or endurance training when our immune system is already weakened.

Of course, you can’t always avoid contact with others completely, but the more consistently you pay attention to it, the lower the risk of getting sick. Without viruses and bacteria to infect you, your body will stay healthy, no matter how susceptible you are after training.

Rule #2: Get Vaccinated

After training, the immune system takes a bit of a dip, making you more vulnerable to pathogens. If your body comes into contact with a virus or bacteria it doesn’t know, it gets twice as tough. The truth is: The only real “boosters” for your immune system are vaccines. No magic powder that your favorite influencer tries to sell you can compare to that.

Stick to the recommended vaccinations for flu, COVID-19, and other diseases. This will ensure that your immune system is optimally prepared, reducing your risk of becoming seriously ill.

Rule #3: Wash Your Hands Thoroughly Before Eating Anything

As we all know, people are real pigs. They touch doorknobs with their unwashed hands, grab handrails, touch touchscreen monitors, shopping carts, cash, and many other things. And when you think about it, you might realize that many of these activities happen right before a meal. For example, you open the front door of a restaurant, order on a touchscreen monitor, maybe even pay with cash at the register, and then sit down to eat – without washing your hands.

We’ve all done this at some point, and probably not always with clean hands, because the situation often doesn’t allow for it.

A little tip for such situations: Having a small bottle of hand sanitizer is worth its weight in gold. It’s always handy and a great alternative to washing your hands in these moments.

And when you do get the chance to wash your hands – do it right. Don’t just rinse quickly under cold water, but wash thoroughly with soap for at least 20 seconds. It’s the easiest and most effective way to get rid of pathogens. (And definitely don’t open the bathroom door with your hand afterward, or it’ll all be for nothing.)

Rule #4: Pay Attention to the Basics

Make sure you're eating a balanced diet with enough protein, healthy fats, and carbohydrates, along with plenty of vegetables, fruits, and enough fluids. If a blood test shows a deficiency in certain vitamins, take targeted supplements.

Carbohydrates play an especially important role, as they not only serve as a vital energy source but also help stabilize blood sugar levels. This, in turn, reduces the release of stress hormones and supports the immune system. Low-carb advocates might want to take this into account.

So, don't be afraid of carbs – they keep your immune system functioning normally!

And most importantly: Pay attention to proper recovery. There’s no point in going full throttle for an hour every day and not allowing your body any rest. That only intensifies the Open Window Effect. Of course, you can train every day and still recover, but the intensity should be managed so that your body always has enough time to recover. So, make sure to get 8 hours of sleep every night and incorporate relaxation methods like meditation and stress management into your daily routine.

You might already be familiar with these and other basics from my main articles on the website, where I go into more detail on topics like nutrition, training, sleep & recovery, and the immune system.

I follow these rules, and I haven’t been seriously sick in over 10 years, despite 5 days of strength training and 2 to 3 cardio sessions per week.

Closing Words

Especially when your training is frequent and intense, rest periods become even more important. If you also pay attention to the basics, like a balanced diet with enough protein and micronutrients, and stay away from sick people, the risk of getting sick during the Open Window Effect is significantly reduced.