Lhasa is not a nightlife city. The altitude flattens the appetite for late drinking, the Chinese authorities take a dim view of foreigners gathering, and Tibetans themselves drink at home rather than in bars. What there is amounts to a handful of cafes good for a daytime stop, a couple of traveller-oriented restaurants with a bar upstairs, and the local Karaoke and dance halls that fill with Chinese soldiers and migrant workers in the evening.
The two reliable foreign-friendly options are Dunya on Beijing East Road for a beer and a Western meal, and Summit Cafe for a competent espresso and somewhere warm to sit. For something more local, look for the small Tibetan-run sweet-tea houses around Barkhor Square — glass after glass of milky sweet tea, often poured from a Thermos, for a few yuan. They serve until early evening but rarely later.