昨日ゆっくり休んだ分、今日は頭が冴えている。

、。

Thanks to having rested well yesterday, my mind is sharp today.

Literal

Yesterday leisurely rested [amount-分], today [topic-は] head [subject-が] is-sharp-[continuous-ている].

~分 attached to a past verb forms a pattern meaning 'to the extent that / thanks to the amount of X' — it transforms a completed action into a quantified basis for a current state. 頭が冴える ('the head is clear/sharp') is a set phrase for mental sharpness. ゆっくり ('slowly, leisurely') here refers to the relaxed quality of the rest.