The science

How VÉLA works

The short version

VÉLA estimates where your body clock is from your roster, then plans sleep, light, caffeine and meal timing to help it move where your next duty needs it. It's built on the Three Process Model of alertness, which describes how your body clock, your build-up of sleep pressure and your grogginess after waking combine to shape how alert you feel.^1,2,3,4 VÉLA assumes your body clock can shift by roughly 1 hour a day when flying east and 1.5 hours a day when flying west,⁶ adjusted for your chronotype, age and feedback. Everything it shows you is an estimate, not a measurement.

What does VÉLA actually predict?

VÉLA keeps track of your body time: what time your body thinks it is, which after a few sectors is often very different from the clock on the wall. It projects that forward through your roster, using your duties, the rest you're likely to get, your profile and anything you tell it after each trip.

From there it estimates two things: where your body clock will be at each point in your roster, and how fatigued you're likely to be. It then plans what to do between now and your next duty to close the gap.

What model is VÉLA based on?

VÉLA is built on the Three Process Model of alertness, a well-established framework from sleep science.^1,2,3,4 It describes alertness as the combination of three things:

Your body clock (circadian rhythm). Your internal 24-hour cycle, with a natural low point in the early hours of your body's night.
Sleep pressure. The need for sleep that builds the longer you're awake and clears when you sleep. Being on duty adds to how quickly it builds.
Sleep inertia. The groggy period just after you wake up, before you're fully alert.

VÉLA applies these principles with its own planning rules, designed around crew rosters. It doesn't use or reproduce proprietary commercial fatigue models such as SAFTE/FAST or the Boeing Alertness Model.

How fast does VÉLA assume your body clock adjusts?

By default, VÉLA assumes your body clock can move about 1 hour per day when it needs to shift earlier (usually flying east) and about 1.5 hours per day when it needs to shift later (usually flying west). These are the average rates in the CDC's jet lag guidance.⁶

They're starting points, not a promise that your body will adapt at exactly that speed. VÉLA also looks at how much time you have before your next duty and how far your body clock needs to move. If there isn't enough time to get all the way there, it tells you, rather than pretending you'll be fully adjusted.

Why is flying east harder?

Shifting your body clock earlier (flying east) is slower than shifting it later (flying west), which is why VÉLA plans a slower rate for eastbound trips.⁶ Going to sleep earlier than your body expects is harder than staying up later.

What does VÉLA personalise?

Chronotype. VÉLA assumes your body clock's low point is around 04:00 body time. If you're an early bird it moves that to around 02:00, and if you're a night owl to around 06:00.
Age. From age 40, VÉLA gradually assumes your body clock shifts a little more slowly, by up to 20% at most.
Sleep preferences. How long you like to sleep and when shape your sleep plan, within what your roster allows.
Your own adjustment limits. If you know you adjust faster or slower than average, you can change the east and west rates.
Caffeine sensitivity. This sets how early your last caffeine should be before sleep (see below).

This is broad personalisation. VÉLA doesn't measure your individual body clock or fit a model to your biology.

How does VÉLA plan your sleep?

VÉLA first works within the rest time your roster actually gives you, and deals with any urgent sleep debt. Then it aims for one main, unbroken sleep near your body clock's low point, adding a nap where it helps. Where you place that sleep also depends on which way your body clock needs to move and how long you have before you're next on duty.

How does VÉLA time light exposure?

Light is the strongest signal your body clock responds to, and its effect depends on timing.^5,6

To shift your clock earlier (usually flying east): get bright light after you wake, and avoid bright light before you sleep.
To shift your clock later (usually flying west): get light before you sleep, and avoid it after you wake.

VÉLA gives you roughly two-hour windows to seek or avoid light, fitted around your planned sleep. It's timing guidance. VÉLA doesn't measure how much light you actually get.

How does VÉLA time caffeine?

VÉLA sets a caffeine cut-off before your next main sleep, so caffeine doesn't get in the way of it. By default that's 6 hours before sleep, or 8 hours if you're sensitive to caffeine and 4 hours if you're not.

How does VÉLA time meals?

Meal guidance follows your body time, not local time. VÉLA suggests avoiding eating and focusing on hydration between about 20:00 and 05:00 body time, then gives breakfast, lunch and dinner windows that fit around your sleep and duties.

To be clear about what this does: research links meal timing to your metabolism and to rhythms in organs like your gut, but not to resetting your main body clock. So in VÉLA, meal times support how you feel and digest. They don't move your body clock estimate. Our Eating Across Time Zones brief goes into more detail.

How does your feedback change the plan?

After a trip, you can tell VÉLA what actually happened:

Sleep: how long you slept, and whether you went to sleep earlier, on time or later than planned.
Light: whether you followed the light advice (none, some, mostly or all of it).

VÉLA then recalculates your body clock, your fatigue estimate and your upcoming plan from what really happened, rather than assuming everything went to plan.

What are VÉLA's limits?

Your body clock and sleep are estimated, not measured. VÉLA doesn't use blood tests, wearables or live sleep tracking.
It assumes the sleep it plans is possible, even when in reality it might be shorter or more broken.
Personalisation is broad (chronotype and age). It isn't fitted to your individual biology.
Real light levels, cabin conditions, medication, genetics and many route-specific factors aren't fully modelled, and all of these can affect how you adjust.⁶
Your feedback improves the estimate, but can't verify your sleep stages, light exposure or true body-clock position.
VÉLA follows the same principles as aviation fatigue management, but it isn't an approved airline fatigue risk management system and doesn't certify fitness for duty.

What research is VÉLA built on?

VÉLA's model draws on the sleep and body-clock research below. Its sleep planning also draws on NASA research on planned rest during long-haul operations (Rosekind and colleagues), research on sleep inertia (Tassi and Muzet) and sleep consolidation (Skorucak and colleagues). Light guidance also draws on Duffy and Czeisler, and meal guidance on Wehrens, Chellappa and Manoogian and their colleagues.

Borbély AA. A two process model of sleep regulation. Human Neurobiology, 1982;1:195–204.
Daan S, Beersma DG, Borbély AA. Timing of human sleep: recovery process gated by a circadian pacemaker. American Journal of Physiology, 1984;246:R161–R183.
Åkerstedt T, Folkard S. Validation of the S and C components of the three-process model of alertness regulation. Sleep, 1995;18(1):1–6.
Borbély AA, Daan S, Wirz-Justice A, Deboer T. The two-process model of sleep regulation: a reappraisal. Journal of Sleep Research, 2016;25(2):131–143.
Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. Journal of Physiology, 2003;549(3):945–952.
Centers for Disease Control and Prevention. CDC Yellow Book: Jet Lag Disorder.

VÉLA provides personal planning insights based on your roster. It isn't medical advice or a medical device, and doesn't replace your airline's fatigue-management requirements. Full details