I have similar experience as you, and what I concluded was that it was the 4-5 hours of looking busy that really made me tired, usually I was surfing the intern

I have similar experience as you, and what I concluded was that it was the 4-5 hours of looking busy that really made me tired, usually I was surfing the internet, switching to a new page every minute. A better strategy is to work 15-30 minutes and then take a few minutes break where you meditate.