The open window theory
For decades, sports scientists have warned of the “open window” effect, a temporary dip in immune function after intense or prolonged exercise, leaving athletes more vulnerable to infections. Classic research by David Nieman and others showed that after 90 or more minutes of hard endurance exercise, immune cells like salivary IgA (your first line of defence against colds) drop for 3-24 hours. This effect is most pronounced after high-intensity sessions or long, gruelling rides, such as VO2 max intervals or 5-hour endurance rides in the cold.
But here’s the twist: Not all exercise suppresses immunity equally. Recent reviews suggest that moderate exercise actually boosts immune surveillance, while only extreme, unaccustomed, or poorly recovered sessions create a meaningful risk. In other words, it’s not just about how hard or how long you ride, it’s about how you balance intensity, volume, and recovery.
Intensity vs volume: which is riskier?
High-intensity intervals (HIIT) and race-pace efforts trigger a bigger immune dip than steady endurance rides. Studies show that after all-out efforts, natural killer cell activity (which fights viruses) can drop by 20-50% for hours, and inflammatory markers rise. But here’s the catch: If you’re well-fuelled, rested, and not already run down, your immune system often rebounds stronger, a process called “immune enhancement.”
Long, slow distance rides are often considered “safe,” but chronic high volume without adequate recovery can be just as risky. A 12-year study of Olympic athletes found that those with the highest training loads had abnormal T-cell responses and more frequent illnesses, especially during winter. The issue isn’t the volume itself, but the cumulative stress on sleep, nutrition, and recovery.
Riskiest scenarios:
- Fasted HIIT sessions (e.g., morning VO2 max intervals before breakfast).
- Back-to-back high-intensity days without recovery.
- 20+ hour weeks with poor sleep or nutrition.
- Back-to-back 4+ hour rides in cold/wet conditions.
Safer alternatives:
- HIIT sessions in a fed state, with carbs before and during.
- Solo outdoor intervals (better air quality, lower pathogen load).
- Cap volume at 80% of your maximum historical load in winter.
- Prioritise sleep and fuelling during high-volume blocks.
HRV, fatigue, and the infection connection
Your body often warns you before you get sick. Heart rate variability (HRV) is one of the best early indicators. Research shows that a sudden drop in HRV precedes illness by 1–3 days in 70% of cases. If you track your HRV, then you can be on the lookout and if it drops by 10–15% below your 7-day average, it’s time to reduce intensity or take a rest day.
Other red flags include resting heart rate 5+ bpm above baseline, poor sleep efficiency (for example scores below 85% on wearables), and subjective fatigue or irritability.
Consistency beats heroics
It is a misconception to label any form of acute exercise as immunosuppressive. Research suggests that instead, exercise most likely improves immune competency across the lifespan. Elite cyclists who complete more than 80% of their planned training weeks are 7x more likely to hit their season goals, largely because they avoid illness-induced disruptions.
Think of your immune system like your FTP. It adapts to stress, but only if you manage the load. The goal isn’t to avoid all risk, it’s to train in a way that strengthens, not suppresses, your defences. Here are the key things to keep in mind.
- Train smart: Swap one HIIT session for a recovery ride if HRV is low. Adjust your training plan around travel and other high stress situations as a precaution.
- Fuel smart: Aim for 60g carbs/hour on rides >90 minutes.
- Sleep smart: Aim for 9 hours on harder training days; nap if needed.
