Japam is of three types Vachika - loud recitation - effective Upamshu - soft recitation - more effective Manasikam - within the mind - most effective. Whether one is physically clean or not, or is walking, talking, eating, sleeping, a true devotee should never stop reciting the bhagawan naama and an upasaka his ishta devata's mUla mantra. But Shuddi and achaara has to be maintained for the first two types of japam. These rules do not apply for mAnasika japa, the superior among the three. Please Note, This is done only mentally and not even a single letter is uttered openly... Ive met people of both extremes.... Not even uttering the name of the upasana devata loudly and also people who chant out moola mantrams loudly sitting on bed(just to show off) Apart from all these... the sastra itself prescribes chanting of certain moola mantrams during night... One classical example is Varahi... "Diva Na Smaret Vaartaaliim" - do not remember varaahi during the day... Devatas like Ucchishta Ganapaty, Pulindini, VanadurgA, Kiratha Sastha etc. can be chanted during the night.. And for a true upasaka there is nothing called "No Result" The sole purpose of japam is to remove the mental attachment towards "Result" and turn it towards the ultimate truth.