。
You should sleep.
Literal
Slept-side [emphatic-が] good [emphasis-よ].
The construction ~た方がいい ('the X-ed side is better' = 'you should X') is the standard way to give advice. Note the verb is in past form — 眠った, not 眠る — even though the action is in the future. Past form here is grammatical, conveying the completion of the action rather than its time. The final よ marks this as friendly advice for the listener.