Each operator is coming up with inventive scheduling, like three days followed by four nights, then seven days off or viceversa.
The simple 7 day rotation, 7 days off, 7 nights rotations, 7 days off no longer seems to apply.
EMS helicopter flying has been around over 30 years, if they have not figured out circadian fatigue is because they really don’t care.
CFR 14 Part 135 is clear when it come to unscheduled on demand crew rest requirements, 10 hours uninterrupted over the last 24 hours.
Day or night you need to show up rested, but in reality many pilots elect to do differently and some even count on bad WX night to get their sleep.
5 flights per day is meaningless per se unless you take into account how many were completed during day or night shift, IFR or VFR, scene calls or inter hospital transfers and of course how long those flights were.
I could hit the border twice or three times a night and those could be 7 hours night unaided or IFR mix, but then once I got home I’d hit the sack at about the right time knowing it may be the same for the next six nights.
It really is up to the flightcrew’s sense of responsibility.