Body clock
Your internal circadian rhythm, which FTL rules recognise through the window of circadian low and which a flying roster constantly fights.
Definition
The body clock is the roughly 24-hour internal circadian rhythm that drives sleep, alertness, body temperature and digestion, set mainly by light exposure. Regulators build it into duty rules: EASA's flight time limitations define a window of circadian low between 02:00 and 05:59 in the time zone you are acclimatised to, and duties that touch it are limited more tightly. Crew rosters — night flights, early sign-ons, time-zone crossings — are essentially a series of collisions with it.
What it means for you
Understanding your body clock explains why a 04:30 sign-on feels worse than a 14-hour day, why a westbound sector is easier than the return, and why a run of earlies followed by a late is the roster pattern crew complain about most. It also tells you when you are legally acclimatised and therefore how long your next duty may be. Protecting it — consistent sleep windows on days off, light discipline on layovers — is the single most effective thing you can do for your energy across a career.
In practice
Under EASA rules the maximum basic FDP of 13 hours applies only to duties starting in the friendly morning window; the same duty starting late at night, into the circadian low, is capped several hours shorter.
Go deeper
Related terms
4 terms- Jet lagCrew lifeBody clock out of step with local time after crossing time zones — the long-haul crew's occupational hazard, and a skill to manage.
- FatigueCrew lifeReduced alertness from sleep loss and circadian disruption — a regulated safety issue with a reporting system you are obliged to use.
- FTLRegulationFlight time limitations — a regulator's legal caps on duty, flight hours and minimum rest: the only hard limit on what a roster can ask.
- Red-eyeRoster & schedulingA flight that departs late at night and arrives in the morning, worked through the body's circadian low; capped more tightly by FTL tables.