Like other recommended acts, nafila prayers are not similar in importance. Therefore, some of them are more emphasized to perform and compensate and some not.  Among the most emphasized and recommended prayers in the holy Quran[1] and traditions are nawafil of the day and night especially the nightly nawafil.
In the holy Quran God addresses His messenger: “And during a part of the night, pray Tahajjud beyond what is incumbent on you; maybe your Lord will raise you to a position of great glory” [2]
Interpreters of Quran have considered this verse as evidence that indicates the outstanding position of night vigil prayers, although this verse does not explicitly suggest it. Several signs are available that make this interpretation seem evident. [3]
A lot of traditions have been received from the infallibles about the merit of night vigil prayers:
1- The Holy Prophet (S) has said: "Mu'men's [practicing Muslim] dignity depends on his night vigil prayers and since on the Day of Judgment, when God brings peoples from both ancient and modern times together, a caller calls those who practiced tahajjud to stand up. Thereupon, a small group stands up. Afterwards, other people would be reckoned." [4]
2- On another occasion, the Holy Prophet (S) has said: "Verily, the houses where night vigil prayers are performed illuminate the heavens as stars illuminate the Earth."
3- There is even another tradition from the Holy Prophet (S) who has said: "Night vigil prayers is a light in the grave for those who perform it." [5]
4- And it is narrated from Holy Prophet (S) that "You'd better practice tahajjud as it is a practice by pious people before you, it leads to closeness to God, it is a compensation for sins and it deters it and it drives the pain away from body.[6]
5- When asked "why those who practice night vigil prayers have the most beautiful faces?" Imam Sajjad ('A) said: "Because they take privacy with God Almighty so He covers them with His light." [7], [8]
6- Imam Sadeq (A) has enumerated some impacts for night vigil prayers: “Night vigil prayers make the face become more beautiful, improve [one’s] temper, purify [one’s] income, [help to] pay back one’s debts, drives away sadness, illuminates [one’s] eyesight. May you perform night vigil prayers which are your prophet’s (s) tradition and they keep away illnesses." [9]
As it is extremely desirable to perform night vigil prayers, it is really praiseworthy to compensate those prayers if they are missed whatever the reason. The Holy Prophet (S) has said: "Verily God Almighty takes pride before His angels over His servant who compensates his night vigil prayers during the day and says: "Oh Thy angels behold Thy servant who compensate those prayers that I have not obliged him, testify that I forgive his sins."[10]
Therefore as it is obligatory to compensate wajib [obligatory] deeds, it is mustahabb [desirable] to compensate mustahabb deeds.
Abdullah ibn Sanan narrates from Imam Sadeq ('A) that he said: "I asked him, how should one compensate his nafila prayers if he has not performed them for a long time and has forgotten the number of them? Imam Sadeq ('A) said: "He should perform as many nafila as he makes sure he has compensated all his nafila prayers…" [11]
Since abandoning mutahabb deeds and prayers is not considered a sin and does not lead to chastisement, if one intends to compensate them, he is free to compensate them at any time.
When asked "I have not said nafila prayers, what time I can compensate them?" Imam Reza ('A) wrote: "Any time you like"[12]
However, it is better to compensate nafila of previous during the coming night and nafila of previous day during the coming day as Imam Sadeq ('A) has said: "If your daily nafila were missed, perform them on the coming day and if your night vigil prayers were missed, perform them on the coming night." [13]
 Muhammad ibn Muslim says: "I addressed Imam ('A): Someone became sick and stopped performing mustahabb deeds and abandoned nafila, what is the ruling in this regard? Imam ('A) said: "O Muhammad! Nafila is not obligatory however, it is better for him to compensate and if he did not, he is not obliged." [14]
 However, it is better and it is mustahabb to compensate mutahabb deeds in order they have been missed; for example in night vigil prayers it is better to first compensate two-rak'at prayers then watr [single rak'at prayer performed at the end of night vigil prayers]. [And if more than one watr have been missed] It is better to perform a two-rak'at mustahabb prayer between watr prayers when compensating.
Imam Baqer ('A) has said: "If one has not performed his watr for two or three or more consecutive nights, he should compensate them in order they have been missed and once he has compensated the first watr he should say at least a two­-rak'at prayer and then compensate the second watr because watr should entail other prayers.[15]

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