Thank you for contacting us. Salat al-Tahajjud is not compulsory, whether you're resident or travelling, but it carries big virtue and great reward. Missing Salat al-Tahajjud does not constitute a sin. One is only missing out on a reward, if it is easy for him to do so, and he finds ample time in his travel schedule as well as a suitable prayer area. However, if it is not easy for him to do so and he does not find ample time in his travel schedule and a suitable prayer area, then it is better for him to leave it, so as not to inconvenience himself and inconvenience his travelling companions. Of the sunnah and optional prayers that the Prophet (peace and blessings be upon him) used to pray even on travel were: Sunnah Salat of Fajr, Salat al-Duha, Salat al-Tahajjud and Salat al-Witr. It is better to pray these prayers and not the other sunnah and optional prayers during travel especially if it is easy to do so and the person used to regularly perform them while resident. However, should performing them prove difficult and hard during travel, and he leaves them off, then there is no sin on him particularly on travel because travel often involves difficulty and hardship and because these prayers are not compulsory like the five compulsory prayers, and Allah knows best.