Fit can also mean a convulsion or spasm, a coughing fit. So fitful is a period full of fits. Hope that helps.

Native speaker and you saved me the effort!

Yeah, "fitful" is a word that sounds good but isn't.

Ha! Same here! I though that it must be something like that link between relief of depression and sleep deprivation. Turns out, fitful is not related to being fit or to a full night of sleep, but more to throwing a fit.

I can see how it'd be interpreted that way at first glance. My read was always that it meant "full of fits", in the way that a child throws a fit or tantrum. So if your sleep is doing that, it's not good.

Not a native speaker, I think I sleep well (feel rested, not tired during the day) but lightly (with typically a few bathroom breaks), should I be concerned?

I am a native speaker and I had to look it up as well.

Interesting how different folks have different experiences for base vocabulary. “Fitful” is a word I’ve probably known since ... heck, I cannot recall when I learned it. Alternatively, I was in my 20s when offered a lasagne for a cough and had no idea what the person was offering. The room looked at me quizzically until I realized it was what I’ve always called a cough drop. Similar with “faux pas” when I first heard it around the same age. And laniard.

it's basically only applied in this context, ie to describe poor or restless sleep