Shafaqna (Shia International News Association) – The Grand Ayatollah Sistani answered some questions regarding Night Prayer. According to Shafaqna the questions and answers are as follows:
Question1: How can we know the time of mid-night? Do 00.00 hours the point of mid-night as it is commonly held by some people?
Answer: Midnight is halfway between sunset and true dawn. So if the sun sets at 7 p.m. and the true dawn begins at 4 a.m., then midnight will be at 11:30 p.m. The criteria for determining midnight are the timings of sunset and true dawn, which differ according to place and season.
Question2: Is it enough for night prayers to read only Salat al-Shif` & al-Witr?
Answer: It is permissible to offer al-Shaf` and al-Witr only. It would also be sufficient to offer al-Witr alone.
Source: Ayatollah Sistani