Travelers to Tibet are reported to exhibit mild symptoms of altitude sickness. Complaints of headache, breathlessness , fever, loss of appetite, unrest, stomach disorder etc can come up before acclimatization. So, proper acclimatization is very much important before you head on trekking trips to a higher elevation. If you choose to take flight into Lhasa, you will need stay about 3 nights in Tsedang or Lhasa area to get properly acclimatized, before going over the high passes. Clothing and important articles recommended. Wind Horse provides a list of appropriate clothing depending on the nature of travel and the areas visited.