English Sentences

Neither of those is 2 sentences as punctuated, although they could be if punctuated appropriately..
The first could actually be 3 standalone sentences, because each of these is complete on its own: "It was the middle of the night. The smoke alarm went off. We woke up frightened." But to have 3 short sentences there is a bit jerky: you may want to alter the rhythm of the passage by replacing one of the full stops with a semicolon. Either:
It was the middle of the night; the smoke alarm went off. We woke up frightened.
Or:
It was the middle of the night. The smoke alarm went off; we woke up frightened.
You can see that the way you use the semicolon affects the relationship between the sentences.
You could do much the same thing by replacing the punctuation with a preposition or a conjunction, thus:

It was the middle of the night when the smoke alarm went off. We woke up frightened. (A semicolon between off and we would also be OK.)
Or:
It was the middle of the night. The smoke alarm went off and we woke up frightened.

The second example is grammatically two complete separate sentences; you could simply put in a full stop after clear. However, since the logical link between the sentences is very strong, a semicolon would be better, thus:
The reason we called you here is clear; we need to make some decisions.

This one should be a colon.