A useful estimate.
Not a crystal ball.

What it is for

An independent travel-planning aid for healthy adults with a regular nighttime sleep routine, making one journey to one destination, direct or with up to three connections. It makes a schedule from the information you enter. It does not diagnose, treat or promise to prevent jet lag.

Seek individual advice for children, pregnancy, sleep disorders, bipolar disorder, eye conditions, photosensitizing medicines, shift work or frequent multi-city travel. Do not use this plan for flight-crew duty, safety-critical work or decisions about whether you are fit to drive.

The calculation, step by step

1. Local times become real instants. Your ticket times are interpreted in each city's IANA timezone and converted to UTC. Impossible spring-forward times are rejected. Repeated fall-back times ask you to choose an occurrence. Arrival must follow departure within 48 hours. With connections, each flight must also land after it takes off and within 24 hours, and each connection must last less than 24 hours.
2. The clock difference is circular. We compare both offsets at arrival, then choose the shorter change within ±12 hours. Exactly 12 hours uses an earlier shift by convention, not because it is best for everyone. A +17-hour wall-clock change can mean a seven-hour later adjustment.
3. Preparation preserves your sleep window. For shifts of at least two hours and stays of at least two days, bedtime and wake time move by up to one hour per preparation day, at most three days and never beyond the needed shift. Preparation days also get light windows. The model assumes you follow this preparation; choose zero days if you cannot.
4. Sleep is an opportunity. On each flight we show overlaps with destination night, leaving 90 minutes after take-off and before landing; time on the ground is never shown as in-flight rest. Each connection shows its local time and whether it falls in destination night or daytime, with a caffeine reminder when a sleep window is less than six hours away and cutoffs are on. When a connection overlaps destination night, a rest opportunity leaves an hour after landing and 90 minutes before the next flight. Connections you do not add count as flight time. After arrival we target your usual bedtime on the destination clock for up to five days. For a stay under two days we project home sleep hours instead.
5. Light windows come from a published body-clock model. We simulate the Forger, Jewett & Kronauer (1999) model of the human circadian pacemaker, with its population-average parameters, after weeks of your usual sleep. Typical light is assumed: 1,000 lux from 08:00 to 20:00, 40 lux in the evening and at night, 50 lux in the cabin and 300 lux in airports, as in a published itinerary model. Each day we find the model's next body-temperature low. Light after it shifts the clock earlier and light before it shifts it later (Khalsa et al., 2003), so up to three hours of brighter light (assumed 4,000 lux outdoors, 300 on board, 1,000 at a connection) start 30 minutes from the low on the helpful side, and up to three hours of gentle light (100 lux, 10 on board) go on the other. Brighter light on the ground stays within 06:00–22:00 and before the two-hour wind-down, and nothing is planned during sleep or rest. The model then simulates that day before planning the next.
6. Large shifts can go the other way round. For shifts of eight hours or more we also simulate the longer direction and use it only when the model predicts it is at least a day faster. Shifts under two hours and stays under two days get no light windows.
7. Adjustment is a model estimate. The day-by-day readings and days to adjust assume you follow the plan. A clock counts as adjusted when its temperature low is within 30 minutes of where it would be on destination time, the criterion used in that itinerary model. Individual clocks, light sensitivity and real light differ, and the model has not been tested on this planner's users.
8. Caffeine is optional. The cutoff is six hours before the scheduled sleep opportunity. No intake or dose is prescribed. Wind-down begins two hours before sleep.
9. Melatonin is not prescribed. A Cochrane review found it reduced jet lag after flights across five or more time zones. Plans for those trips mention it, but never a dose or timing: ask a doctor or pharmacist whether it suits you.

What the model does not know

Your measured circadian phase, chronotype beyond your entered sleep routine, light sensitivity, the light you actually get, daylight or weather, medical history, sleep debt, missed preparation, flight delays and airport transfers are not modeled. A connection under 24 hours is treated as part of one journey toward your destination: the planner does not assume your body clock adjusts to a stopover's time zone. A stopover of 24 hours or more, or a return journey, needs a separate plan. This is not Timeshifter's algorithm and has no claimed equivalence to it.

Daylight saving can alter the elapsed length of a generated sleep window. Advisory events in a missing local hour move to the next valid minute; repeated hours use the first occurrence. Calendar exports carry UTC instants so they remain unambiguous.

Do not sacrifice safe visibility or necessary sleep to follow a schedule. If symptoms persist or seriously affect daily life, seek medical advice.

Sources and editorial responsibility

The SoloSquare team designed the rules and interface. The sources below support general principles, not the effectiveness of this specific tool.

• CDC Yellow Book: Jet Lag Disorder — sleep shifts, circadian timing, direction of adaptation and caffeine timing; published 23 April 2025.
• NHS: Jet lag — travel rest, destination routines and practical advice for short trips; page reviewed 31 May 2023.
• Forger, Jewett & Kronauer (1999), A simpler model of the human circadian pacemaker — Journal of Biological Rhythms 14(6):532–537; the model and parameters used for light timing.
• Khalsa, Jewett, Cajochen & Czeisler (2003), A phase response curve to single bright light pulses in human subjects — Journal of Physiology 549:945–952; light before the temperature low delays the clock, after it advances it.
• Burgess, Crowley, Gazda, Fogg & Eastman (2003), Preflight adjustment to eastward travel — Journal of Biological Rhythms 18(4):318–328; shifting sleep and morning light before departure.
• Huang, Bin, Caillaud & Postnova (2026), Modeling the effects of flight itinerary on jetlag duration — Sleep 49(6):zsag063; typical cabin, airport and daylight levels and the 30-minute adjustment criterion.
• Herxheimer & Petrie (2002), Melatonin for the prevention and treatment of jet lag — Cochrane Database of Systematic Reviews, CD001520.
• IANA Time Zone Database — civil-time rules used through Python's zoneinfo. Governments can change future rules.

Source pages checked 18 October 2026. Research and implementation were AI-assisted; no clinician endorsement is implied. Report corrections to [email protected].

Your itinerary stays yours

The server calculates the plan in memory, without saving it to a trip database or attaching it to your account. Tool pages load no analytics or advertising scripts. Normal server security logs may record an IP address and the public page path, but the itinerary is sent in a POST body, not in that path. Downloads stay on your device. Optional share links contain the itinerary in a URL fragment; recipients can read it.

The optional check-in after your trip stores only your answers, the day and a rounded trip type (clock change in whole hours, number of connections and stay length), never your cities, flight times, name or IP address.