When you start yawning widely, you know it's time to get your rest. Which word is the possessive adjective?

Possessive adjectives are also Forms of Possessive Pronouns.

Her, his, your, my, its, our are the most common possessive pronouns used as possessive adjectives. Adjectives always come before the noun or pronoun.

The pronouns "you" and "it" are being used as a nominative pronouns. The only other pronoun in this sentence is "your" which in this case is a possessive pronoun.

Since possessive pronouns can also be possessive adjectives, "your" is the possessive adjective.