Ueli Steck once told me that athletes are the ones most bothered by the impact of high elevation. Not because they are more prone to its effects, but because they are typically more tuned into their body’s sensations and more sensitive to changes. Because athletes are accustomed to performing well, these changes in the body can be seen as reason for alarm when they are usually little more than a warning to simply slow things down and acclimate.
Headaches, poor sleep patterns, elevated heart rate, decreased appetite, lethargy, and a general feeling of being in slow motion are all normal when going to elevation. For a trail runner traveling to the Himalaya, how high the effects of elevation are felt will be different for everyone, but typically start at around 3500 meters.
If you want to run at this elevation, first you need to spend some time acclimatising, and not running. In popular places like Nepal’s Khumbu Valley, where much of the time is spent between 4000-4800 meters, you might need to spend 4-5 days just to get to 4000 meters to insure that the body adjusts accordingly. And this is just to settle in, going for a proper trail run may take a few more days of going higher still, but coming back down.