ثَلاثٌ لا يَهْنَأُ لِصاحِبِهِنَّ عَيْشٌ : الحِقْدُ ، وَ الْحَسَدُ ، وَ سُوءُ الْخُلْق
Three traits you cannot live in happiness if you have them: hatred, envy and ill manners.
سهر الليل في طاعة الله ربيع الأولياء وروضة السعداء
Staying awake at night to worship Allah is springtime for holy persons and a garden for the happy.
Worshiping Allah at night means performing (صلاة الليل); which roughly translates to (prayer of the night). This prayer is not mandatory for Muslims but it is highly regarded and is considered one of the best ways to perfect the soul and get closer to Allah.
Imam Ali himself performed this prayer regularly. And since he knows its benefits for the yearning soul, he recommends it for all worshipers and seekers of the truth.