Lo único que obtendrás es un mal descanso nocturno.
Why is lo used here?
El or la is used in this context.
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In this case lo is the neutral article (determiner):
el → masculine singular
la → feminine singular
lo → neutral singular
los → masculine plural
las → feminine plural
All of these Spanish articles correspond to the form THE in English.
In your example there is nothing determining the gender of what is the only thing that you will obtain, therefore you use the neutral.
La única COSA que obtendrás es un mal descanso nocturno.
El único RESULTADO que obtendrás es un mal descanso nocturno.
lo is normaly used when there is no noun determining the gender of the subject; anyway the concordance with the adjective should be done with the masculine (of the adjective).