Excess sleepiness can be a sign of increased toxicity or side-effect of detoxification....that's my experience. I can get very sleepy sometimes though not necessarily more so during rounds.
I believe it's to do with a decrease in oxygen levels and circulating toxins, both affecting the brain. Perhaps it's a sign you need to support your liver more, do some bowel cleansing eg enemas, fibre and maybe take more antioxidants.