Question

At which stage of nrem sleep will you feel most groggy if you are suddenly awakened from it?

Answer 1

I believe the answer is: Stage 4 of NREM sleep

During the stage 4, people would experience the deepest and the most powerful dreams throughout the sleeping process.
When people are suddenly waken up in this situation, they tend to exprience distortion in their mind because part of their brain still adjusting between the dream and the reality.