Meaning trap

brighten up

become brighter or happier

to start looking or feeling happier, or to become lighter and sunnier (about the weather).

phrasal verbB1common

Don't mix these meanings

Same phrase, different job.

become brighter or happier

to start looking or feeling happier, or to become lighter and sunnier (about the weather).

brighten up (make brighter or happier)

Another meaning of 'brighten up' is 'make brighter or happier'; compare the examples to keep the meanings separate.

Notice where the phrase sounds natural.

She really brightened up when she saw her friends at the party.daily life
The weather started to brighten up in the afternoon, which made everyone feel more energetic.IELTS speaking

Pattern: subject + brighten up

WrongShe brightened up herself when she heard.

RightShe brightened up when she heard.

In this sense 'brighten up' is intransitive — no object. (Cheering someone else is 'brighten up their day'.)

Patternsubject + brighten up

Choose the version that sounds natural in speech.

Stiff Her facial expression became noticeably more cheerful.

Natural She really brightened up.

'Brighten up' is the natural verb for a mood lifting (or weather clearing); the formal version is stilted.

Words that usually travel with it.

Compare with 'cheer up', which is only about mood, not light.

brighten up (make brighter or happier) — Another meaning of 'brighten up' is 'make brighter or happier'; compare the examples to keep the meanings separate.