Tahajjud prayer is an optional/supererogatory prayer performed during the night after waking up from sleep, because the meaning of tahajjud is to give up hujud which means sleep, that is, to give up or leave off sleep.

Many Muslim jurists use tahajjud to mean prayer performed during the night in the general sense, that is, after or before sleeping. The time of Tahajjud prayer is after one wakes up from sleep and before the adhan of Fajr prayer.
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