When they reached Darchen at night to stay in the guest house, the protagonist wasn't able to breathe properly. This can be attributed to the high altitude of the place, as Darchen is located at an elevation of approximately 4,575 meters above sea level. The lack of oxygen at such high altitudes can cause breathing difficulties, especially for those who are not acclimatized to the conditions.

The high altitude of Darchen can lead to a condition known as altitude sickness or acute mountain sickness (AMS). AMS occurs when the body is unable to adjust quickly to the reduced oxygen levels at high altitudes. Symptoms of AMS include headache, dizziness, nausea, fatigue, and difficulty breathing.

In the case of the protagonist, the lack of oxygen caused difficulty in breathing properly. This could have been due to the thinner air and reduced oxygen levels at such a high altitude. The body needs time to acclimatize to the lower oxygen levels, and this process can take several days.

It is important to note that altitude sickness can affect individuals differently, and some people may be more susceptible to its effects than others. Factors such as age, physical fitness, and previous experience with high altitudes can also play a role in determining the severity of symptoms.

To alleviate the symptoms of altitude sickness, it is recommended to rest, stay hydrated, and avoid strenuous physical activity. In severe cases, descent to a lower altitude may be necessary to ensure proper oxygen intake.

In summary, when they reached Darchen at night to stay in the guest house, the protagonist experienced difficulty in breathing properly due to the high altitude of the place. This can be attributed to altitude sickness or AMS, which occurs when the body is unable to adjust quickly to the reduced oxygen levels at high altitudes.