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 in this context.
In Spanish, when expressing a recommendation, suggestion, or necessity, as indicated by "Es importante que" (It's important that), the verb that follows should be in the subjunctive mood. The correct form should be "duermas." Therefore, the correct sentence is "Es importante que duermas 8 horas cada noche." This use of the subjunctive conveys a sense of purpose or uncertainty about the action.