A word there is.....

Well well well. A whole week and still no takers.
This one must be hard.

Prince/s ??

I agree with kev but he left out the second part: prince becomes princes which is pural, add another s and it becomes princess which is singular once again

There are other words like this: handle, pa, bra, saltine, bible, deadline...

Silly people. The answer is cares / caress.

Can you explain why cares fits better than princes, or deadlines, or pas, or bras?

Gloo

It is obvious that "cares" answers all the clues in the riddle (see the second and last lines). The other "answers" have little to do with this riddle.

Well, that explains it. It's obvious! Why didn't I see that before?

"Foe to ease and tranquil slumber" - don't you think that deadlines can do this?

"And sweet what bitter was before." - ever heard the phrase "sweet death"? Don't you think some people consider their lives bitter.

I'm not even going get into whether you and your dad sleep with your bras on.

Well done Eos, you beat all the regulars to the punch.
Gloo....um....chill out a bit man.