Q:  Salams, when one intends to pray Tahhajud do they have to sleep before praying it? Or can one pray it at 12AM if awake?
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A:  Tahajjud time starts after you have prayed your isha so you can pray it at anytime after that until Subh Sadiq 
It has 3 stages 

A’alaa, Awsat, and Adnaa 
A’alaa is to pray after waking up 

Awsat is to pray before going to sleep 

And Adnaa is to pray immediately after isha 
Do whatever is easy for you