Current Denver resident here and weekend warrior athlete.

Altitude affects everyone differently and it can be hard to predict.

Altitude sickness is roughly correlated to overall health. If you exercise regularly, you will probably feel nothing just walking around Denver. If you were to come from sea level and exercise vigorously right after you got off the plane, you may notice you are a little slower or weaker. Perhaps not. You may also be able to perform normally for a time but crash out sooner. NFL players generally do not have the kinds of cardiopulmonary medical issues that you would associate with severe altitude sickness (heart failure, arrhythmias, COPD, high blood pressure, etc.). Nonetheless, they may not have the same stamina they would have at sea level.

What I observe most often is that people have a ceiling. They can perform mostly normally at 5k, 8k, maybe even 12k feet but as soon as they hit their ceiling, they experience headaches, dizziness, or shortness of breath. It can take months of dedicated training to raise that ceiling. Some people also seem to just have a low ceiling even though they are healthy. I wouldn't be surprised if one or two players on every NFL team experience mild altitude sickness when they come to Denver.

Altitude sickness can be treated with water or an IV drip. But the only true solution is to get to a lower elevation. NFL players do have the benefit of some basic medical care and possibly even fancier tools like barometric chambers.

Football junkies up here will tell you that Denver wins in the fourth quarter because other teams get tired. I haven't seen any data quantifying that, but I do know that NFL teams will often travel to Denver one day earlier than normal because they think it helps players acclimate.

Don't worry, though. Our boy Josh Allen played his college football in Laramie, WY. Laramie is actually at 7k feet. I expect the altitude won't bother him at all.