Tahajjud has been especially cultivated in the Sufi orders and among the devout of every school as the private engine of the spiritual life, distinct from the public and obligatory character of the five daily prayers. Manuals of the inner life treat the capacity to keep the night watch as a fruit of long training, since it depends on both waking and inward attentiveness. Because it follows sleep, it is distinguished in the books from a continuous vigil kept without rest.
The practice connects to the wider Islamic esteem for the night as the time of nearness to God. In Ramadan its communal counterpart is the tarawih; outside it, tahajjud remains the individual's own, unregulated by congregation and unseen, which is exactly the quality the tradition prizes in it.
Al-Ghazali, in the Ihya Ulum al-Din, treated the night vigil as one of the surest engines of spiritual advancement precisely because it is unseen and unrewarded by any worldly recognition, arguing that a deed performed when everyone else is asleep carries a sincerity public worship cannot fully replicate; later Sufi orders built formal litanies and recommended numbers of rak'ah around the practice, while insisting, in keeping with the wider tradition, that the exact number matters far less than the wakeful intention behind it.