Personally I would not have agreed to that early a start time in the first place unless I was willing to do it 'regularly' because sadly one thing I have learned over the course of my career is that the minute you say yes to 'special' before you know it it creeps into being 'normal' for the client ... so I do not do special!

Personally if I had concern this was going to cause burn out and so forth I would just say 'sorry but our initial agreement was X and I am happy to stick to that but if you cannot than we are going to sever the arrangement ~ I am willing to provide service during the notice period provided you are able to stay within the agreed upon hours ... otherwise they would loose their deposit for 'altering' the contract!