The same flight on the same aircraft can carry a different maximum duty depending on one invisible factor: where your body clock thinks it is. Acclimatisation is how the rules account for the fact that a crew out of step with local time tires faster.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Why the body clock is in the rules
Human alertness follows a daily rhythm, and that rhythm is anchored to the time zone you are used to, not to the clock on the wall wherever you happen to land. Fly far enough east or west and your internal clock and local time drift apart, so a duty that local time calls mid-afternoon may fall in your body's small hours. Fatigue rules build this in, because ignoring it would let a roster look legal on paper while asking a crew to perform at their physiological worst.
The window of circadian low
The sharpest expression of this is the window of circadian low (WOCL), the part of the night, roughly 0200 to 0559 in the time zone you are acclimatised to, when the body is least alert. Both major systems treat duty through the WOCL as more demanding. Under EASA's ORO.FTL.205, a maximum flight duty period is reduced when the duty encroaches on the WOCL, and the whole structure of report-time reductions is shaped around it. The key point is that the WOCL is measured against the time zone you are acclimatised to, which is why acclimatisation has to be defined first.
How EASA defines an acclimatised state
EASA ORO.FTL.105 defines an acclimatised crew member as one whose circadian clock is synchronised to the local time where they are, and treats you as acclimatised to a band 2 hours wide around the local time at your point of departure. That band is your reference time.
What happens next is the part almost everyone gets backwards, so it is worth stating flatly. The guidance to ORO.FTL.105 says that a crew member remains acclimatised to the reference time for 47 hours 59 minutes after reporting, no matter how many time zones they have crossed. You do not fall out of the acclimatised state by flying east. You fall out of it by staying away.
From 48 hours, the regulation's Table 1 takes over. It compares the time difference between your reference time and the local time where your next duty starts against the time elapsed since you reported at reference time, and returns one of three states:
- B, acclimatised to the local time of the departure time zone: your reference zone. This is the state for the whole of the first 48 hours, for every size of crossing the table covers.
- D, acclimatised to the local time where the next duty starts: the new zone, once enough time has passed there.
- X, an unknown state of acclimatisation: the transition, and it cannot begin before 48 hours.
How long the unknown state lasts scales with the crossing, not with the direction. Under 4 hours of time difference there is no unknown state at all: you go straight from B to D at 48 hours. Up to 6 hours of difference, you sit in X from 48 hours and reach D at 72. Up to 9 hours, D arrives at 96 hours; up to 12 hours, at 120.
The unknown state matters because it carries a reduced maximum FDP, read from a separate, shorter table in ORO.FTL.205: 11 hours for one or two sectors, falling to 9 hours by six, against a basic maximum of up to 13 hours for an acclimatised crew. But state B is not a free pass either, because while you are acclimatised the maximum FDP is read against your reference time, not the wall clock in front of you. Report at 0800 local in a zone five hours ahead of London and, if London is still your reference, the table is entered at 0300 and the FDP is cut accordingly. The clock the rule uses is the one in your head.
How the FAA handles it
The FAA achieves a similar result with different words, in 14 CFR 117.3:
- A theater is a geographical area in which the flight duty period departure and arrival points differ by no more than 60 degrees of longitude, about four time zones.
- A crew member is acclimated when they have been in a theater for 72 hours, or have been given at least 36 consecutive hours free from duty.
When operations move across more than 60 degrees of longitude, the crew is no longer acclimated to the new theater, and the rest and duty provisions adjust accordingly until they re-acclimate. As ever, the FAA figures and the EASA figures are not interchangeable; they are two schemes solving the same problem.
A worked example
A crew based in London reports at 0900 London time and flies east to a city five time zones ahead, landing in the evening. Their reference time is London, and by ORO.FTL.105 they stay acclimatised to it for the next 47 hours 59 minutes, however far east they have flown. So the next day's duty out of that station is not read from the unknown-acclimatisation table. They are still in state B, on the acclimatised table, but the table is entered at their reference time. An 0800 local report there is 0300 in London, which is why a duty that looks like a civilised morning start can carry the maximum FDP of an early-hours one.
It is the third day that changes the state. Once 48 hours have elapsed since that London report, Table 1 moves a five-zone crew into X, the unknown state, with its reduced maximum FDP of 11 hours for one or two sectors. They stay there until 72 hours have elapsed, at which point they become D, acclimatised to local time, and the full table returns.
Under the FAA scheme, the same crew would not be acclimated to the new theater until they had spent 72 hours there or been given 36 consecutive hours free of duty. The dates and numbers differ, but in both cases the rule is buying back safety margin for a body clock that is out of step, and in neither case does it assume the clock moved with the aircraft.
Eastward versus westward, and recovery
Not all time-zone crossings are equal, and the asymmetry is worth understanding because it shapes how quickly you re-acclimatise. The body clock runs on a cycle slightly longer than 24 hours, which makes it easier to delay than to advance.
- Flying west lengthens your day. You stay up later and the body clock has to delay, which it does relatively comfortably. Westward trips tend to be easier to adapt to.
- Flying east shortens your day. You have to go to sleep and wake earlier than your body wants, advancing the clock, which it resists. Eastward trips are generally the harder direction, and the fatigue from them lasts longer.
A common rule of thumb is that the body re-synchronises at roughly one time zone per day, and more slowly eastward than westward. So a crew that has crossed five zones to the east may take the better part of a week to fully adapt to the new local time. Worth noticing, though, is that the regulation does not model that asymmetry at all. EASA's Table 1 keys only on the size of the time difference and the hours elapsed; it does not ask which way you flew, and it declares you acclimatised to the new zone at 48, 72, 96 or 120 hours whether the crossing was east or west. The FAA's 72-hour theater rule is equally direction-blind. The physiology is asymmetric; the rule is a blunt calendar. That gap is yours to manage, which is what the FAA's acclimated definition and the EASA provisions leave to you rather than solve for you.
This is also where fatigue risk management comes in. Long-haul and complex time-zone operations do not always fit neatly inside fixed-hour rules, so both ICAO and the authorities allow an approved Fatigue Risk Management System (FRMS) as an alternative, where an operator manages fatigue with data and mitigations rather than fixed limits alone, to an equivalent level of safety. The reference is ICAO Doc 9966, the Manual for the Oversight of Fatigue Management Approaches. For the individual pilot, the practical takeaways are simpler: expect eastward trips to hit harder, give recovery real time rather than assuming you have adjusted, and plan sleep deliberately around the window of circadian low in whatever zone your body is still keeping.
A worked multi-day trip
Trace a trip to see how the state actually moves. A London-based crew reports at London and flies west across five time zones on day one. On arrival they are, correctly, still acclimatised to London time, and by the EASA table they stay that way: for the first 47 hours 59 minutes they are in state B, and any duty they fly out of the western station is read from the acclimatised table at their London reference time. Because they have flown west, that reference time runs ahead of local, so a comfortable-looking local report can land in an unfavourable reference-time band. The table, not the wall clock, decides.
At 48 hours they pass into the unknown state, with its reduced maximum FDP of 11 hours for one or two sectors, and at 72 hours they become acclimatised to the western local time and the full allowances return. Note the order carefully: the reduced allowance arrives on day three, not on arrival. It is the price of the transition, not of the flight.
Then they fly home, eastward, five zones the other way. Because they had become acclimatised to the western time before departing, that western local time is now their reference time for the trip home, and the clock starts again from the report there: 48 hours before the state can move at all.
Under the FAA, the same trip turns on whether they are acclimated to each theater: 72 hours in a theater, or 36 consecutive hours free from duty, before the local allowances apply, with more than 60 degrees of longitude marking the move to a new theater.
The point the worked trip makes is that acclimatisation does not track the aircraft. It tracks the clock: 48 hours before the state can change at all, and up to 120 hours before the largest crossings settle. Reading it as "we crossed five zones, so we must be in the unknown state" gets it wrong in both directions at once, inventing a reduced FDP on day one and forgetting the real one on day three.
Common pitfalls
- Assuming you fall into the unknown state on arrival. You do not. Under EASA you stay acclimatised to your reference time for the first 47 hours 59 minutes, however many zones you crossed; the unknown state cannot start before 48 hours.
- Measuring the WOCL against local time. It is measured against the zone you are acclimatised to, not the wall clock where you landed.
- Reading the acclimatised table against the wrong clock. In state B the maximum FDP is entered at your reference time, not at the local time where you report, and the two can be many hours apart.
- Assuming you acclimatise instantly to the new zone. Re-synchronising takes time, and the table only credits it at 48, 72, 96 or 120 hours depending on the size of the crossing.
- Carrying one scheme's thresholds to another. The EASA acclimatisation table and the FAA 60-degree theater and 72-hour rule are separate.
- Treating fatigue as only about hours. Direction and number of time zones crossed matter as much as the raw duty length.
In Pilot EFB
Pilot EFB can track acclimatisation across an imported roster and flag where a duty runs through the window of circadian low, giving you a readout of how your body clock is likely sitting against the schedule. It is a planning and awareness aid that estimates these states to help you see the fatigue picture; it is not a compliance system, and the binding determination of your acclimatisation state and maximum FDP rests with your operator's approved scheme and the current regulation. Pilot EFB is not a certified Electronic Flight Bag. Saved roster data stays available offline; pulling a fresh roster needs a connection.