Is there a definite time for voluntary night Salah (Prayer)
The entire night is a proper time to offer voluntary Salah. However, the time of Witr (Prayer with an odd number of units) is due only after `Isha' (Night) Prayer, whereas Tahajjud (optional late night Prayer) is best performed, if possible, in the last third of the night. If a person fears that they may not wake up during the last part of the night, it is preferable to stay in the safe side and offer Witr in the beginning of the night. The Prophet (peace be upon him) said:
Anyone who fears that they may not get up in the last part of the night, let them offer Witr in the first part of it; and anyone who is eager to get up in the late part of night, let them offer Witr at the end of the night, for Salah at the end of the night is witnessed (by the angels) and that is better.
(Part No. 7; Page No. 221) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family and Companions.