La fiebre empeora por la noche.

The fever gets worse at night.

Breakdown of La fiebre empeora por la noche.

• la noche: the night
• empeorar: to get worse
• por: at (as in 'at night')
• la fiebre: the fever

Questions & Answers about La fiebre empeora por la noche.

Why is it la fiebre and not just fiebre or una fiebre?

• As a subject, Spanish typically uses the definite article with conditions and body-related nouns: La fiebre empeora...
• After tener, you normally drop the article: Tengo fiebre (I have a fever).
• Una fiebre is possible when you specify/type it: Tiene una fiebre alta; but for everyday speech, tengo fiebre and, as a subject, la fiebre are more idiomatic.

Is fiebre feminine? How do I agree adjectives with it?

Yes, fiebre is feminine, so use feminine agreement: la fiebre alta, fiebre leve, fiebre persistente (not: alto/leveo/etc.).

Why the simple present empeora and not está empeorando?

Spanish uses the simple present for habits and general truths. Por la noche signals a habitual pattern, so La fiebre empeora por la noche is natural. Use está empeorando when you mean “is getting worse right now”: La fiebre está empeorando.

Do I need the reflexive form (se empeora)?

No. La fiebre empeora is the most common and neutral. The pronominal form exists but is less usual here; many speakers would prefer another verb with se, e.g., La fiebre se agrava.