1) don’t need to sleep, be able to sleep: the coolest thing in the universe.

2) don’t need to sleep, be unable to sleep: you would likely cope. Also, be an irreplaceable shift worker.

3) need to sleep, be able to sleep: this is where most of us are now, nothing to write home about.

4) need to sleep, be unable to sleep: the rest of your life is short and full of exponentially growing suffering.

By „don’t need to sleep” I mean that the body refreshes itself by different means and you still reap the benefits.

I can’t really imagine how it would feel, much like I can’t imagine being immortal. Most fiction describing those phenomena as „nightmare” usually is about a character who gains the ability but none of the protective mechanisms.