Never, ever, ever, ever, ever let your clients dictate your working hours.
Set your daily work hours, just like a business should and always does, and close your doors at the time.
Mine are 11:00 to 19:00. This seems kinda weird but I deal with clients in different timezones so overlap is needed.
However, I only work 3-4 hours a day and am available during the rest of the day for client emails, notifications and actual work relating to my business.
Again, set your own hours and be your own boss. That's one of the reasons you work for yourself, right?