What do Alice and Elisabeth both suffer with in the novel, What Alice Forgot?

Both Alice and Elisabeth suffer from insomnia, a trait that they inherited from their mother. Alice and Elisabeth always used to talk about it and to them it represents a sisterly connection, something that they share that nobody else outside of their family would understand. Alice asks Elisabeth about her insomnia after losing her memories, but Elisabeth says they hadn’t spoken about it in some time, representing how fractured their relationship had become.