Xining sits at 2,270 metres above sea level — high enough to affect some visitors on arrival, particularly those flying directly from sea-level cities. Mild symptoms (headache, fatigue, mild shortness of breath on exertion) are normal and usually pass within 24 to 48 hours. Serious altitude sickness is rare at this elevation but possible, particularly in older travellers or those with heart or respiratory conditions.
The practical advice: rest on the first afternoon, drink more water than usual, avoid alcohol for the first day, and don't exert yourself heavily. Xining's role as an acclimatisation stop before heading to higher elevations — Yushu at 3,700 metres, Lhasa at 3,650 metres — is one of its most important functions as a transit city. Spend two nights here before ascending; it makes a significant difference to how you feel at altitude.