the first version of the hos was enacted in 1938, and four revisions have been made since. the more recent revisions have relied on research into the human circadian rhythm (the tendency for humans to follow a natural 24-hour cycle with 8 hours of uninterrupted sleep). as of july 1, 2013, a driver operating under federal property carrying regulations is limited to 11 hours of actual driving within a 14-hour period, and requires a 30-minute break during the first 8 hours of on duty time. after which he/she must rest for 10 hours. the rules do not explicitly require that a driver must sleep, only that a driver must take a period of "rest" within the sleeper berth or off duty (i.e., at home). distinct regulations exist for passenger carrying drivers as well as vehicles designed specifically for oilfield operations as well as construction related activities. some us states also allow for special intrastate regulations to be followed if the vehicle is operating entirely within the bounds of the given state.