This literally just began happening to me a couple of weeks ago. Now, I live northern enough that currently there's only 7½ hour daylight, so most of the day it's actually dark, and for these nights it doesn't really make much difference having those first and second sleep, but it is weird.
I turn 43 later this year, but I also started some new anti-anxiety medication around the same time back when it began happening, so maybe it's just temporary until I figure out dose and when to take the medicine.