@nprofile1q... I researched this for my site and found a plugin that could do it. But it only worked on the blog post page. This was a couple of years back, so maybe things have changed.
I'm not a technically-minded person and I think in the dark days when I set up my site, I just chose a theme that doesn't support a lot of things. :)
Might look into it again though, with something like the content I offer, I can see how it might be useful to someone who doesn't want to disturb their sleeping partner.