It is said that when the mind enters Nibbana, the body and mind process ceases(Nama-rupa nirodha). This non-experience sounds somewhat similar to what we experience in deep, dreamless sleep(continuously arising Bhavanga cittas) where one does not have a sense of time and existence until one awakes in the morning.I've heard that there are about 5 to 6 ways that a meditator might mistake the mind entering such prolonged states of Bhavanga as Nibbana. Ex: due to extreme Viriya(energy) the mind enters prolonged Bhavanga states during meditation. Another is due to heightened Passadhi(calmness).This is said to be explained in detail in one of the books by venerable Mahasi Saydaw. I'm looking for the book and the section where this is explained.Thank you!