Anglicans are the same, yes. But both Catholic and Anglican nuns are under obedience to their mother superior, and can be required not to participate in the full programme of prayers, or can be given permission to eat meat during Lent, or whatever.
My cousin was an Anglican nun and she had a standing permission to carry money with her for buying biscuits when she went to visit my gran, because they knew my gran didn't feed her enough! 
But also, the elderly nuns in her order who weren't well enough didn't have to get up in the night, and they didn't have the same restrictions on what they ate. I don't know when those rules came into force in Anglican orders and it may be things were more strict in the 1950s, but essentially, the monastic Rules set out in the early centuries of the Church are pretty good at accepting people may need to be let off physical duties if they can't cope.
What's the Book of Hours in this context? Is it a book about Catholic convent life, or do you mean the Book of Hours in the liturgical sense?