The sun vanished over the horizon

Alternatively: The sun disappeared beyond the horizon.

Die Vorschläge von Carly und Norbert treffen es super - hier einer, wie man es auch formulieren kann...
'the sun dips below the horizon' - 'die Sonne versinkt am Horizont'

If one doesn't want to stick too closely to the original "verschwinden", another possibility would be:

The sun set on the horizon

I actually like Carly's proposal a tad better than mine.

Norbert - You really SHOULD get some rest! Stiff deadline??

No deadlines right now. But whenever I work a lot, I start time shifting (i.e. I get up later and later, which naturally means I also go to bed later and later), and once I have shifted I may need more than a weekend to "unshift".