"I am not sleeping, because it is daytime."
Translation:Nie śpię, ponieważ jest dzień.
In this context it would be really weird. There's no real "it" to refer to, "right now" it is daytime.
Well I can imagine that one may say that... but why? It's more likely to hear „wieczorowa pora” or ”pora nocna”, but I've never heard „pora dzienna” even if it is valid phrase. We shouldn't calque everything into polish. Besides it is long. „pora dzienna” vs „dzień”.