I assume you are refering to Montague's speech in Act I scene 1 when he comments to Benvolio how concerned he is about Romeo because of the way he seems to be languishing in life. Montague uses a number of strong images, but perhaps the following are worthy of note:
Many a morning hath he there been seen,
With tears augmenting the fresh morning's dew,
Adding to clouds more clouds with his deep sighs...
Note the way that hyperbole is used to express this father's concern about his son. He says that Romeo is so morose that every morning he addes to the dew with his tears, and likewise his sighs make more clouds because of his sadness. Then, when the sun comes up and light and warmth are brought to the world, Romeo does the following:
...private in his chamber pens himself,
Shuts up his windows, locks fair daylight out,
And makes himself an artificial night.
Romeo is so depressed that he is unable to face sunlight and the joy of the day, and so continues his night-time vigil by making a form of perpetual night in his room during the day.