What role does the "lo" in this spanish sentence play?

Question

¿Por qué no duermen lo suficiente los adolescentes?

I know "lo" can mean The; how; it; him; the one.

But what does it mean here and why is it necessary, if it is necessary.. and why doesn't it say suficientemente instead.

Answer 1

But what does it mean here and why is it necessary, if it is necessary..

It's necessary because, by itself, the word "suficiente" is an adjective. In the sentence given, the expression "lo suficiente" acts (grammatically) as an adverb and works (semantically) to specify "how much" sleep adolescents get.

The term for such usage is "lo referencial." To help understand the way in which "lo referencial" is being used here, we can paraphrase our original as follows:

Original:

¿Por qué no duermen lo suficiente los adolescentes?—Why don't teens sleep enough?

Paraphrase:

¿Por qué no duermen los adolescentes lo que es suficiente dormir?—Why don't teens sleep what (the amount that) is adequate to sleep?

Essentially, the expression "lo suficiente" allows us to pose the question, "Why don't teens get a sufficient amount of sleep."

thanks this makes sense! - kuddah, Dec 8, 2013

Answer 2

My oppinion: 'Suficiente' usually needs an adjective or a noun to refer to. So, with verbs you have to say 'lo'.
'Suficientemente' is used with adjectives. 'Suficientemente grande' for example.

Answer 3

Suficiente is an adjective and therefore must appear with the noun that it's modifying. If the noun is an abstract idea without gender you use the neuter definite article lo in front of the adjective.