Method and sources

How Evenwake builds your plan, what it was checked against, and what it can't do. Updated September 28, 2026.

Travel and wellness guidance, not medical advice. Evenwake does not diagnose or treat anything. Check with your doctor if you have a health condition, are pregnant or take medication. Not for pilots or cabin crew on duty.

1. The body-clock model

Evenwake simulates your body clock with the Jewett-Forger-Kronauer model: a mathematical oscillator driven by light, published in 1999 (Jewett et al., J Biol Rhythms 14:493–499; Forger, Jewett and Kronauer, J Biol Rhythms 14:533–538). It is the model behind two reference studies on jet lag schedules: Dean, Forger and Klerman 2009 (PLoS Comput Biol) and Serkh and Forger 2014 (PLoS Comput Biol 10:e1003523).

We use the 7th-order version with the parameters printed in Table 1 of two open-access PLoS One papers (2019, 2021), which also appear in the supplementary Text S1 of Serkh and Forger 2014:

μ 0.13
q 1/3
k 0.55
p 0.5
α0 0.05
β 0.0075
G 33.75
I0 9,500 lux
τx 24.2 h
k_c 0.4

The body-temperature minimum (the low point of the clock, used to time light) is taken as the model's minimum plus 0.97 h. That offset is not printed in the original papers we have; it comes from an open-source implementation by the same research group, and our own check supports it (see below).

2. How it was checked

Serkh and Forger 2014 publish, in their Figure 1, the daily body-temperature minimum for 7 light schedules after a 12-hour time shift. We extracted those points from the figure's vector data (not by eye) and ran the same schedules through our model.

In 125 of the 126 daily points compared (7 schedules × 18 days), our model lands within 6.5 minutes of the published point.
For the number of days to partial and complete adjustment, it matches the figure in 14 of 14 cases.
Three cells of that paper's Table 1 don't match its own Figure 1; we test against the figure.

This checks that the model is implemented correctly against a published source. It is not a trial in people.

3. How the plan is chosen

The model's clock starts from your own sleep hours, whatever they are: sleeping from 10 p.m. to 6 a.m. puts the body-temperature minimum near 3 a.m., and from midnight to 8 a.m. near 5 a.m. (Roach and Sargent 2019, Front Physiol 10:927). For each night of your trip, the app simulates your clock and tries different lengths of light exposure, keeping the one that brings your clock closest to the destination time. It simulates both directions (advancing and delaying) and keeps the faster one. If your trip has a return flight, it also compares adapting to the destination with staying on home time, and keeps the one with less total misalignment, with one rule: the trip home must never come out slower than with no plan (0.05-day margin). Light levels used in the simulation: 1,000 lux when you seek light (an overcast day outdoors or bright indoor light), 100 lux indoors, 5 lux when you avoid light, and 0 lux asleep.

Rule	Source
Seek light after the body-temperature minimum to advance the clock, and avoid it before	Burgess et al. 2003, J Biol Rhythms 18:318
Seek light before the minimum to delay the clock	Eastman et al. 2005, Sleep 28:33
Before you fly, move your sleep 1 hour a night toward your destination's time, for 0 to 3 nights: earlier when flying east, later when flying west	Eastman et al. 2005; Burgess et al. 2003; CDC Yellow Book 2026
Before flying home, prepare at your destination: 0 to 3 nights moving sleep 1 hour toward home time, chosen by simulation	CDC Yellow Book 2026; Eastman and Burgess 2009, Sleep Med Clin 4:241
Coffee is fine from waking until 6 hours before sleep	CDC Yellow Book 2026; Eastman and Burgess 2009
No coffee in the 6 hours before bed	Drake et al. 2013, J Clin Sleep Med 9:1195 (400 mg of caffeine taken 0, 3 or 6 hours before bedtime disrupted sleep)
Why evening coffee matters when advancing: caffeine 3 hours before bed delayed the clock by about 40 minutes	Burke et al. 2015, Sci Transl Med
Short naps (about 25 minutes) to stay alert, only when needed	CDC Yellow Book 2026 (“Short naps (~20 min) may help with alertness”)
Trips shorter than 2 days: stay on home time	CDC Yellow Book, “Jet Lag Disorder” (Figure 7.4.1)
Sunrise and sunset at each airport	NOAA, General Solar Position Calculations
Melatonin timing (only if you say your doctor or pharmacist recommended it, only where the App Store country allows it, never a dose)	Burgess et al. 2010, J Clin Endocrinol Metab 95:3325

4. Optional tools, and where each one comes from

All of these are optional. Some rest on a published study; others are practical choices we made, and we say which. When the model estimates something (days, hours of shift, a color), it is an estimate, not a measurement of you.

Tool	What it does	Where it comes from
On board	One card per flight: set your watch to your destination's time; when to sleep, or that it's night at your destination midway through the flight; the meal (unless you choose to skip it, the plan keeps the first 90 minutes and the last 60 minutes of flights over 5 hours free of sleep); when to open or close the window shade and which side of the plane the sun comes in; and go easy on alcohol and coffee, and drink water.	Destination clock, window shade, alcohol, coffee and water: CDC Yellow Book 2026; night at your destination: Eastman and Burgess 2009; the sun's position: NOAA. Not verified: that window light alone is enough to move your clock, and our sun-side calculation against NOAA's own calculator. The meal-service minutes come from a review we haven't checked: a product choice.
Mealtimes	Breakfast when you wake, lunch 5 hours later and dinner 4 hours before bed, by the clock where you sleep (by your home clock on short trips). Eating on schedule helps your digestion keep time; it doesn't move your main body clock.	Wehrens et al. 2017, Curr Biol 27:1768; CDC Yellow Book 2026. The exact mealtimes are a product choice.
Exercise times	“If you exercise, now is best” from 13 to 16 h of your body's day when your clock needs to move earlier, or from 19 to 22 h when it needs to move later, and “avoid hard exercise” in the other window. With the extra option: also around 7 to 8 h of your body's day when moving earlier, and a note for your sleep type.	Youngstedt et al. 2019, J Physiol 597:2253 (60 minutes of exercise; large advances at 7 a.m. and from 1 to 4 p.m.); Thomas et al. 2020, JCI Insight (52 young adults: in early risers, evening exercise shifted the clock 0.41 h later; in late sleepers, morning and evening exercise shifted it earlier). Approximations: the hour of your body's day is estimated, and where “early” and “late” sleep types split is a product choice. How much exercise helps without light isn't established.
Light gear	With a light box, light after sunset is simulated at 3,500 lux. Light glasses only change the wording: we found no published light level for them, so they're simulated like bright indoor light. Without dark glasses, “avoid light” means staying indoors in dim light.	Light box: Eastman and Burgess 2009; light box or light glasses after sunset: Roach and Sargent 2019. Assumptions, not verified: the light level of light glasses, and 5 lux for dark glasses.
Light in shorter sessions	30-minute sessions with 30-minute breaks instead of hours in a row. In the model it takes longer: Santo Domingo → Madrid needs 3.9 instead of 2.9 days with continuous light (7.0 with no plan).	Burgess et al. 2003 (no significant difference between intermittent and continuous light); Eastman and Burgess 2009
Clock change	A short plan for the daylight-saving change where you live, made with the same model.	Model only: there's no study on how fast to shift for a clock change. Measured with our 1-hour yardstick, the change adjusts in about 0.1 days with or without the short plan, so it gains nothing measurable; we show it so you know what the change does to your clock.
Important moments	The plan keeps its light off them. If a moment falls in your estimated low-energy hours (within 3 hours of your body's low point), the plan suggests coffee in the hour before and a 25-minute nap when they fit, and shows what moving the light costs in days.	Low-energy hours: Roach and Sargent 2019; coffee and naps: CDC Yellow Book 2026. Protecting moments is a product choice, not a study.
Plan B for each light block	To seek light: go outside in the sun, sit by a sunny window, a very bright indoor space, your light box, or wait for the next block. To avoid it: dark sunglasses or dim light indoors. Each option shows how much that block alone would move your clock, estimated with the model.	Light levels of 100, 1,000 and 10,000 lux: Julius et al. 2019, PLoS One; light box at 3,500 lux: Eastman and Burgess 2009. The window has no published figure: its light level is an assumption, and the app says so.
Things you want to do	Green, yellow or red, depending on how much it would move your clock against the plan, whether it overlaps the plan's sleep, or whether it falls in your low-energy hours. If it isn't green, the app looks for a green time within 6 hours.	Model estimate. Where the colors change is a product choice.
Your stops	On a trip through several cities (a city counts as a stop if you stay 24 hours or more), the whole trip is simulated at once, and at each stop the plan measures whether adapting to local time or keeping your previous schedule works better. With more than 3 stops, it uses the 2-day rule.	Model; 2-day rule: CDC Yellow Book. That this works better in real life has not been studied.
Your body clock now	From your last 7 nights of sleep in Apple Health, the model estimates when your body's low point falls, give or take 2 hours. Always labeled “estimated, not measured”.	The margin: “Finding DLMO” 2020, Chronobiol Int (with 7 days of sleep data, limits of agreement of about ±1.8 and ±1.9 hours). The estimate itself comes from our model.
Light you got (Apple Watch)	For days that have ended, the minutes in daylight your Apple Watch counted; the plan recalculates with them. The watch counts minutes, not brightness, so each minute is counted as 1,000 lux: an assumption.	Estimating the body clock from light measured at the wrist: Stone et al. 2021, Sleep (in night-shift workers). The lux level is our assumption.
Time a day to get ready before you fly	Before each departure, the light to seek each day stays within the time you say you have (no limit, or 0 to 4 hours). Less time can mean more days to adjust.	Product choice.
Warm shower	A warm shower or bath 1 to 2 hours before bed at your destination, even for 10 minutes (water at 40–42.5 °C). It doesn't move your body clock; it helps you fall asleep.	Haghayegh et al. 2019, Sleep Med Rev (review and meta-analysis of studies)
Sleep more before you leave	If you can, sleep more in the nights before you leave: by going to bed earlier when your clock needs to move earlier, or by sleeping in when it needs to move later, so you don't block the light that moves it.	Rupp et al. 2009, Sleep 32:311: a week with 10 hours in bed meant fewer attention lapses in the short-sleep days that followed. A laboratory study, not a study of travelers. How to add the sleep is our rule.
Free time on board	Each awake stretch of 30 minutes or more on board, in destination time.	Product choice (information only).
“How much jet lag do you feel today?”	One 0–10 rating a day in your arrival diary.	Liverpool jetlag questionnaire study, Chronobiol Int 2020 (460 air passengers): a single rating “may be useful for capturing the subjective experience of jetlag”. The full 15-question scale is not used.
NOW, full screen	Black and amber: one big instruction, until when, how long is left, and the next change.	Product choice.
Protect your sleep	From landing, when the plan has you sleeping within the next 12 hours: a sleep mask and earplugs block light and noise, and a Focus keeps notifications quiet.	Practical option, with no health claim.

5. What the model estimates, in examples

Days from landing until the model's clock is within 1 hour of the destination and stays there. These are model estimates for made-up but realistic flight times, not measurements in people.

Trip	Time shift	With the plan	Abrupt shift, no plan
New York → London	+5 h	1.8 days	4.8 days
Santo Domingo → Madrid	+6 h	2.9 days	7.0 days
New York → Tokyo	−11 h	8.6 days	18.5 days

6. Limits we know about

The model is a model: your own clock may be faster or slower, and it doesn't know your light, stress, alcohol or illness.
On round trips, the plan has a rule: the trip home must never come out slower than with no plan (0.05-day margin). In our 12 round-trip tests (Santo Domingo ↔ Madrid and New York ↔ London, stays of 1.5 to 7 days), no trip home comes out slower than with no plan beyond that margin. The case that used to lose (Santo Domingo ↔ Madrid, 5-day stay) now ties: 5.7 days with the plan and 5.7 without it.
Clock changes (daylight saving) are counted: the target is the local clock time. On a New York ↔ London trip that crosses the U.S. change on November 1, 2026, that alone adds days to the trip home.
The cabin is assumed to be 100 lux, and dark glasses or an eye mask 5 lux; we have no published figure for either.
After the flight, melatonin timing comes from the model's clock, not from a measured marker.
Alerts depend on iOS, which decides when they are delivered.

7. What we tested and left out: your age

Evenwake does not ask your age, and here is why. A study comparing young adults (18–31) with older adults (59–75) found that, relative to the melatonin rhythm, the point where light switches from delaying the clock to advancing it comes about 1.8 hours earlier in older adults; the authors describe the statistical evidence as marginal (Kim, Duffy et al. 2007, J Circadian Rhythms 5:4). We tried it in our model. Moving the plan's light by that amount made the plan worse, not better: New York → London went from 1.8 to 2.9 days, and New York → Tokyo from 8.6 to 12.6. The study has no data for ages 40 to 59. What does change with age in that study, going to bed earlier, is already covered: the plan starts from the sleep hours you give it.

8. Why this page exists

Apple's App Review Guideline 1.4.1 asks health-related apps to disclose the data and methodology behind their claims and to remind people to check with a doctor. This page is that disclosure. If you find a mistake, please write to support@evenwake.com.