Brain fatigue?

Lately, when I'm doing office work, I feel like my brain just shuts down as the evening approaches. I feel like my brain is tired. Is this because I'm getting older? Back in my 20s, I felt like I could do endless office work and thought leaving right on time was a waste. I did feel body stiffness from staying in the same posture, though. Now, I feel both body stiffness and a sense of my brain refusing to think, so my afternoon work isn't productive…😩 Will I become less capable of thinking as time goes on?