How does the body adapt to high altitude and when does acclimatization occur?

Your body starts producing more red blood cells roughly from the first day at altitude, but noticeable improvement in how you feel usually comes around the third or fourth day - which matches your experience. I was in the Altai at around 2500 meters and I remember how on the first day I couldn't sleep properly at all because of rapid breathing, and then my body just kind of switched over. The main thing during this period is not to overexert yourself and drink plenty of fluids - that way adaptation happens faster.

I remember the first time I went up to 2500 meters and didn't expect it to be so physically demanding. I thought it was just exhaustion from the hike, but no - even lying in my tent in the evening, it felt like I wasn't getting enough air. The second day was the worst, then it gradually got easier. What's interesting is that the most annoying moment wasn't at altitude, but when I was coming down - my body had already adjusted, and suddenly the normal oxygen level seemed excessive.

To add to what's already been said: besides the increase in red blood cells, your body also restructures your breathing. In the first few days you breathe faster and deeper, but that's tiring and causes dizziness. When acclimatization is happening naturally, breathing becomes more efficient, less panicked. This happens alongside the production of new blood cells, so the improvement isn't instant - it's gradual.

Plus a lot depends on how fast you go up and individual characteristics. If you ascend slowly, giving your body time at each stage, the symptoms will be milder. But if you fly in a plane straight to 3000+ meters, even someone used to mountains can feel pretty rough for the first couple of days.