Will my night prayer become invalid if I am mid prayer and it is time for Fajr? Can one make Qadha for the night prayer? And are there any Duas which could help me wake up easily for it?
1.Your Night Prayer (Salat Al-Layl) is valid even if time of Fajr starts while you are still praying Night Prayer.
2.Night Prayer can be performed as Qadha and it is recommended for those who missed it to perform it as Qadha any time during day or night.
3. Last verse of Sura Al-Kahf has been mentioned in our books to be recited to wake up easily قُلْ إِنَّمَا أَنَا بَشَرٌ مِّثْلُكُمْ يُوحَىٰ إِلَيَّ أَنَّمَا إِلَٰهُكُمْ إِلَٰهٌ وَاحِدٌ ۖ فَمَن كَانَ يَرْجُو لِقَاءَ رَبِّهِ فَلْيَعْمَلْ عَمَلًا صَالِحًا وَلَا يُشْرِكْ بِعِبَادَةِ رَبِّهِ أَحَدًا
الكهف ١٨ | الآية: ١١٠]Sura 18, verse 110.
Wassalam.