Only about half the word appears in the topic list and I was like... "No, it couldn't be..."
But it is.
I admire your honesty at least. But no, you do not discuss this with clients.
You've received some good advice. Adjust your schedule if possible so your workload is lighter during these times. Otherwise, if it's really debilitating, treat it like any other illness or extenuating circumstance, but be prepared for clients to lose patience if this happens more than once on a project.
I would really focus on workload management. This applies to any situation where there may be times you cannot work. Sometimes I travel and I can do work but my time is limited, so I put in extra time before the trip to get as caught up as possible, for example.
I do think it is reasonable, if you have a long-term client, to tell them you have a chronic illness that occasionally makes it very difficult for you to work for short stretches of time. Then they can decide if they can live with it or not. It doesn't matter what the illness is, and I doubt most clients want the details.