If an adjective has 2 syllables only and also ends in a ‘y’, then we remove the ‘y’ and add -ier.
Sleepy → Sleepier → I am sleepier than earlier.
happy → happier → I am happier than ever!
angry → angrier → I am angrier than last time!
For all other adjectives with 2+ syllables, we add more before the adjective.
Interesting → more interesting → This class is more interesting that that one
stressful → more stressful → This project is more stressful than ever
exciting → more exciting → That ride was more exciting than that one.