"Sleepy" is a correct adjective and can be used in the position.
Adjective and adverb
If you've ever wondered why she sings beautifully sounds right but she sings beautiful sounds wrong, you've bumped into the adjective-vs-adverb split. Pick the wrong one and the sentence sounds clearly off — even though native speakers couldn't always tell you the rule. Getting this distinction reliable is what makes descriptions land instead of stumble.
Adjectives describe nouns: a fast car, the soup is hot. Adverbs describe verbs, adjectives, or other adverbs: he drove fast, unbelievably smart. The same word can flip roles — fast covers both — but most words don't.
Adjective
If you've ever written a French nice old wooden table and felt something was wrong without knowing why, you've hit the adjective-order rule. English insists on a particular sequence — opinion, size, age, shape, colour, origin, material — and rearranging the words makes a sentence sound non-native even when every individual choice is correct.
An adjective describes a noun or pronoun: a tall building, the soup is hot. Most adjectives also take comparative and superlative forms (taller, tallest), which is how you compare things — another core piece you need from day one.