I agree that going for a walk - and above and beyond getting enough sleep when possible - are good strategies.

I also try not to eat a lunch that is heavy in carbohydrates as when I do I am more prone to sleepiness.

The worst time I ever had with this was when I was pregnant. It was later felt that my thyroid had probably been out of whack but none of the professionals at the time thought of that. I was so sleepy it was a nightmare in the afternoons. Most of my clients I would arrange to see in the morning or very late afternoon which seemed better. One could only come at 2pm and no matter what I did the fatigue would hit me. I was so tired I actually had tears rolling down my face (no way of passing it off as empathy) and would drape my legs over the side of another chair. A few times I caught myself wondering with serious intent if it would be TOO distracting for the client if I lay on the floor as she talked. Was just able to stop myself from asking her....

The odd thing was she did not seem to mind and just talked as if nothing was happening. I apologized and encouraged her to talk about what that was like for her, but she said she was (and truly appeared to be) unaffected. She saw me since well before my pregnancy and knew I had not been like that then, so maybe that helped her to not personalize it - I don't know.

It rarely affects me now, but when it does I generally try to acknowledge to the client that I didn't get enough sleep or whatever so that hopefully it feels less personal. I think generally clients will notice anyways so it's good to acknowledge it. If I find I have a run where a particular time of day is bad for me, I avoid booking during that time period when possible.