How does one know if the reason for missing the night prayer was a sin he committed or due to charity he gave?

Qiyam (definition) al-Layl (Night Vigil) is among the best acts of obedience, and whoever abandons sins and adheres to acts of obedience will be aided in performing it. Al-Hasan (definition) said, "Your sins have shackled you," to one who was exhausted by Qiyam al-Layl. Whoever commits to Qiyam al-Layl and is then overcome by sleep after having intended to perform it, it is hoped that he will attain the reward for his intention, and it is legislated for him to make up his missed portion during the day. A servant's deprivation from performing Qiyam may be due to a sin he committed, so he must hold himself accountable, repent, and reform.