One has to get up at night and go to sleep with a strong desire to perform tahajjud; It has been said in various hadiths that the reward of tahajjud will be obtained even if one does not wake up at night. It was narrated from Abu Ad-Darda' (RA) who attributed it to the Prophet (ﷺ): "Whoever goes to his bed intending to get up and pray qiyam at night, then sleep overwhelms him until morning, will have recorded that which he intended for and his sleep will be a charity given to him by his Lord."

Reference: Hasan (Albani). Nasai: 1787