The US EMS industry generally works the 7/7 roster....7 days, 7 Off, 7 nights.
The night shift is a killer....in more ways than one. Long about day five...you are tired...day six...you are really tired...and day seven just cannot end fast enough.
Night shift starts with interrupted sleep....I know of no one that stays on night shift when off duty as we all try to lead somewhat normal lives and that means awake days...sleep nights.
The trick to surviving night shift is to try to start your shift with a "Sleep Credit" by taking a good long nap before arriving for that first shift and planning your time between shifts so you sleep as long as possible and arise just prior to going to work. Arriving at work with a sleep deficit particularly on day one sets the stage for a very tiring week.
The 2/2/2 roster described by some sounds good....except you are rotating shifts rapidly but at least you are not on duty but the two nights in a go.
Perhaps a 4/4/4 is closer to the right answer....as you get a good long break away from work and plenty of time to rest. Fatigue can be cumulative...and that is the dangerous part. You arrive at work tired and don't realize it.
Anything longer than the 4/4/4 would set you up for the same problems as the 7/7/7 in my view.
The fallacy of the nine hour shift is you now work more days in a row....and rotate between three shifts. The longer you can be away from work for a real break is better in my view.