Let’s just rip the bandage off and get at the heart of the discussion. If you feel tired shortly after you start working, then you may be exerting a significant amount of mental energy with your coworkers.Much like your physical exertion can exhaust you over time, mental exertion can exhaust you too.
Most people start feeling tired around the end of a long work day. This is normal. You have used a hefty amount of mental energy making various decisions throughout the day.