Light and darkness

The midnight sun is a natural phenomenon in which the sun is above the horizon for more than 24 hours.
Then how is called the phenomenon in which the sun is below the horizon?

Answer:

Have you ever experience the midnight sun?
I never have...
Oh well, it's just a simple question, isn't it?
Read the question carefully,
and precisely understand what is asked.
I didn't say "below the horizon for more than 24 hours"...

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