Window of circadian low (WOCL) 0200 to 0559 and pilot fatigue
Checked against the sources below on 29 October 2026. How these guides are written
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The window of circadian low (WOCL) is 0200 to 0559. The FAA defines it in 14 CFR 117.3 as "a period of maximum sleepiness that occurs between 0200 and 0559 during a physiological night"; EASA (ORO.FTL.105) and CASA (CAO 48.1) use 02:00 to 05:59 in the time zone you are acclimatised to. Duty in that window shortens your limits: an FAA unaugmented flight duty period starting between 0000 and 0359 is capped at 9 hours, against 14 hours for a 0700 start.
- FAA WOCL
- 0200 to 0559 "during a physiological night"
- Acclimated time
- 14 CFR 117.3
- EASA WOCL
- 02:00 to 05:59
- Time zone you are acclimatised to
- ORO.FTL.105(28)
- CASA WOCL
- 0200 to 0559; infringed by any duty in that window
- Acclimatised location (App 2) or where duty starts (other appendices)
- CAO 48.1 s 6.1, 6.2
- FAA night-start FDP
- 9 h for a start at 0000 to 0359, any number of segments; 14 h at 0700 to 1159 (1 or 2 segments)
- Unaugmented, acclimated time
- Part 117 Table B
- FAA consecutive nights
- Up to 3 FDPs in a row in the WOCL; 5 with a 2-hour rest opportunity in suitable accommodation each night
- Consecutive FDPs
- 14 CFR 117.27
- EASA disruptive duties
- Night duty touches 02:00 to 04:59; early start 05:00 to 05:59 (early type) or 06:59 (late type)
- Acclimatised time
- ORO.FTL.105(8), (9)
- EASA FDP extension
- Max 5 sectors with no WOCL, 4 if WOCL encroached by 2 h or less, 2 if more
- Planned 1-hour extension
- ORO.FTL.205(d)(3)
What is the window of circadian low?
It is the stretch of the night when your body clock makes you sleepiest and least capable, and all three authorities fix it at 0200 to 0559. The FAA's definition in 14 CFR 117.3 is "a period of maximum sleepiness that occurs between 0200 and 0559 during a physiological night". EASA's ORO.FTL.105(28) defines it as "the period between 02:00 and 05:59 hours in the time zone to which a crew member is acclimatised". CASA's CAO 48.1 (section 6.1) uses 0200 to 0559 local time where you are acclimatised, and section 6.2 says the WOCL "is infringed" if any duty falls in any part of it.
The clock that counts is your body clock, which the rules approximate with acclimatisation. Under Part 117 you are acclimated after 72 hours in a theater (an area within 60 degrees of longitude) or after 36 consecutive hours free from duty. Under EASA you stay acclimatised to the local time of your reference time for 47 hours 59 minutes after reporting, however many time zones you cross (GM1 ORO.FTL.105(1)).
How the flight time rules use the WOCL
Part 117 Table B cuts the maximum unaugmented flight duty period for night starts: 9 hours for any start from 0000 to 0359 regardless of segments, 10 hours from 0400 to 0459 for up to 4 segments, and 12 hours from 0500 to 0559 for up to 4, against 14 hours for a 0700 to 1159 start with 1 or 2 segments. Section 117.27 limits you to three consecutive flight duty periods that infringe the WOCL, or five if each night includes a rest opportunity of at least 2 hours in suitable accommodation. A long-call reserve assigned to a duty that begins before and runs into your WOCL must get 12 hours' notice of report time (117.21(d)).
EASA's ORO.FTL.205 Table 2 gives an acclimatised crew 13:00 for one or two sectors starting 0600 to 1329, but 11:00 for a start between 1700 and 0459. A planned extension is limited to 5 sectors when the WOCL is not encroached, 4 when it is encroached by 2 hours or less, and 2 when it is encroached by more (ORO.FTL.205(d)(3)). EASA also defines night duty separately: a duty period encroaching any part of 02:00 to 04:59, which is narrower than the WOCL.
These are the regulatory minimums. Your operator's approved scheme, its fatigue risk management system and your own fitness for duty sit on top. Part 117.5 requires you to report rested and to state that you are fit for duty before each flight.
Why overnights and early starts are hard on the body clock
Early starts cut the sleep you get before duty. Roach and colleagues (Accident Analysis and Prevention, 2012) tracked 70 short-haul pilots with wrist activity monitors: sleep in the 12 hours before duty averaged 5.4 hours for starts between 04:00 and 05:00 and 6.6 hours for starts between 09:00 and 10:00, about 15 minutes lost for every hour the start moved earlier than 09:00. Self-rated fatigue at sign-on was worst for the 04:00 starts.
Late finishes cost alertness at the end of the duty instead. In a study of 95 short-haul pilots (Journal of Sleep Research, 2022), sleep before early starts averaged 6.94 hours against 8.47 hours before late finishes; pilots felt worse before early starts and worse after late finishes, and the authors conclude that every flight encroaching on the biological night deserves fatigue risk management.
Flying through the whole night is worse again. Sallinen and colleagues (2017) followed 86 short- and long-haul pilots over almost two months. In duty periods covering the whole home-base night (00:00 to 06:00), about one in three had a reduced sleep-wake ratio and one in two had severe sleepiness (Karolinska Sleepiness Scale 8 or 9) in at least one flight phase. For duties with no night time the figures were about one in 30 and one in 36.
Local time, UTC and the body clock
Your logbook records UTC or local time, and neither is your body clock. EASA's rules assume you stay on home time for about the first two days after reporting, so your WOCL can fall in the afternoon or evening at a long-haul layover. The rules handle this with acclimated or reference time, so a 0300 local report in a new time zone may not count as a night start if you are still on home time, and a midday report may land in your WOCL.
For an FTL question, use the acclimatisation table in your operator's scheme. For a sleep or fatigue question, talk to your operator's fatigue management team or your AME (FAA), AeMC or AME (EASA) or DAME (CASA).
Mistakes pilots make
- Counting the WOCL in the local time of a layover city you have not acclimatised to. The FAA uses acclimated time and EASA and CASA use the time zone you are acclimatised to.
- Treating EASA night duty and the WOCL as the same window. Night duty is 02:00 to 04:59; the WOCL runs to 05:59.
- Assuming the WOCL only matters if you are airborne in it. CASA counts any duty in the window, and the FAA and EASA limits depend on when the duty period starts and what it overlaps.
- Treating early starts as the easy option. A 04:00 start cost about 1.2 hours of sleep against a 09:00 start in the Roach study.
- Reading a legal roster as proof you are rested. Part 117.5 puts the fitness-for-duty statement on you, and FTL limits are the floor.
What Off Chocks tracks
- With Health turned on, Off Chocks counts each month's overnights (airborne time overlapping 02:00 to 05:59 in the departure airport's local time), early starts (pushback before 07:00 local), late finishes (arrival at or after 23:00 local) and night hours, with the flights behind each count.
- It works from your logged block and airborne times only. It does not know your duty periods, report times or the time zone you are acclimatised to, so it is not a flight time limitations calculator, a fatigue model or a fitness-for-duty check. Your operator's FTL scheme or FRMS is the reference.
Common questions
- What time is the window of circadian low?
- 0200 to 0559. The FAA (117.3), EASA (ORO.FTL.105(28)) and CASA (CAO 48.1) all use that window.
- Is the WOCL in local time or UTC?
- Neither directly. It is in the time zone you are acclimatised to (EASA, CASA) or acclimated time (FAA), which after a time-zone change can still be home time.
- What is a physiological night's rest under Part 117?
- 10 hours of rest that includes 0100 to 0700 at your home base, or at the acclimated location if you have acclimated to a different theater (117.3).
- How many nights in a row can I fly under Part 117?
- Three consecutive flight duty periods that infringe the WOCL, or five if each includes a rest opportunity of at least 2 hours in suitable accommodation (117.27).
- What counts as an early start under EASA FTL?
- A duty starting 05:00 to 05:59 (early-type schedule) or 05:00 to 06:59 (late-type schedule) in acclimatised time, under ORO.FTL.105(8).
- Do early starts reduce sleep?
- Yes. Short-haul pilots slept 5.4 hours before 04:00 starts against 6.6 hours before 09:00 starts, about 15 minutes less per hour earlier (Roach et al., 2012).
Sources
- 14 CFR Part 117 Flight and duty limitations and rest requirements (eCFR)
- EASA Easy Access Rules for Air Operations: ORO.FTL.105 Definitions and ORO.FTL.205
- Civil Aviation Order 48.1 Instrument 2019, Compilation No. 3 (F2021C01239)
- Roach GD et al. Duty periods with early start times restrict the amount of sleep obtained by short-haul airline pilots. Accid Anal Prev 2012
- Arsintescu L et al. Early starts and late finishes both reduce alertness and performance among short-haul airline pilots. J Sleep Res 2022
- Sallinen M et al. Sleep, alertness and alertness management among commercial airline pilots on short-haul and long-haul flights. Accid Anal Prev 2017
This is general information from the published research and regulators, not medical advice. For your own situation, ask your aviation medical examiner, an occupational physician or your operator's radiation protection officer. Any estimate in Off Chocks is for your own awareness, not a dosimeter reading.