I was once told that birds that have the largest eyes sing before dawn. Apparently, the large eyes enable the bird to absorb more light, and thus they can see quite well in relative darkness.
I thought a Robin as well. They can sing all through the night. Here's an interesting piece from the RSPB.
//Singing is hard work, and uses hard won food reserves, so it is the fittest, best-fed males who produce the strongest, most impressive song. Females therefore choose a mate who sings best, because such a male is more likely to be good at raising chicks, to have a good territory, or to pass successful genes to their young. \\
I have an app on my phone that identifies birds from their birdsong. I love it as quite often you can’t see the birds but you can hear them singing away and it’s lovely to start learning how to identify them.