I'm always to tired😩

My SO wants to be intimate almost every night and once I lay down my body automatically goes into a state of sleep. I'm so tired at night and when he tries to start something I get mad and then he gets mad. We have sex about once a week. I don't understand why I am so tired. I work full time as a legal secretary and he also works full time as a ranch hand. We are both 21. How do I become more intimate and less tired at night? Anyone else have this problem?