The verb tense in the sentence "Es importante que dormes 8 horas cada noche" is incorrect because "dormes" is in the present indicative tense, which is not suitable for this context. In Spanish, when expressing something important or subjective with "es importante que," the subjunctive mood should be used instead.
The correct form should be "Es importante que duermas 8 horas cada noche," where "duermas" is the present subjunctive form of the verb "dormir."