〜たほうがいい: you had better

Recommends a specific action as the better choice.

Past form + ほうがいい. Negative uses ない-form without past: 〜ないほうがいい. Can sound pushy toward superiors; soften with ですよ or use 〜たらどうですか. The た here is not about the past: it names a concrete action as the better choice, so 寝たほうがいい means "you had better sleep". After the た-form the ほう of ほうがいい can read as "side": the option on your side.