Your body runs on a near-24-hour clock
The human circadian pacemaker, located in the suprachiasmatic nucleus of the brain, maintains a rhythm that is remarkably close to 24 hours. Research on humans isolated from time cues found that the average intrinsic period is 24.18 hours, with very little variation between individuals.
This consistency means your body clock drifts by only about 11 minutes per day when left on its own. But it also means that when you cross time zones, your clock does not simply snap to the new schedule. It has to be shifted deliberately.
Czeisler CA, Duffy JF, Shanahan TL, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999;284(5423):2177-2181.Light is the primary signal
Light is the strongest signal that shifts the circadian clock. But the effect of light depends entirely on when you see it. This relationship is called the phase response curve.
A single pulse of bright light at night can shift your clock by up to three hours. The direction of the shift depends on timing: light in the early biological night causes a delay (your clock moves later), while light in the late biological night causes an advance (your clock moves earlier).
The phase response curve to light. Exposure in the early night delays the clock; exposure in the late night and early morning advances it.
The pivot point: circadian temperature minimum
The circadian temperature minimum (CTmin) is the point where your core body temperature reaches its lowest value, typically 1 to 2 hours before your natural wake time. This is the pivot point that determines how light affects your clock.
- Light before CTmin delays your clock (makes you wake later)
- Light after CTmin advances your clock (makes you wake earlier)
JetShift estimates your CTmin based on your sleep patterns and uses it to time all recommendations. Get bright light at the wrong moment and you shift in the wrong direction.
Zeitzer JM, Dijk DJ, Kronauer RE, Brown EN, Czeisler CA. Sensitivity of the human circadian pacemaker to nocturnal light: melatonin phase resetting and suppression. J Physiol. 2000;526(Pt 3):695-702.Why eastward travel is harder
When you fly east, you need to advance your clock. When you fly west, you need to delay it. Because the human clock runs slightly longer than 24 hours, delaying is easier than advancing.
A typical person can delay by about 90 minutes per day but advance by only about 60 minutes per day. This asymmetry means that a six-hour eastward trip (New York to Paris) takes longer to adapt to than a six-hour westward trip (Paris to New York).
For eastward trips of more than eight time zones, it can actually be faster to delay all the way around (treating it as a westward shift) than to advance. JetShift calculates the optimal direction for every trip.
Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin. 2009;4(2):241-255.Melatonin: timing, not dosing
Melatonin is a hormone your body produces in darkness. Taking it as a supplement can also shift your clock, but like light, the timing matters more than the amount.
The phase response curve for melatonin is roughly opposite to light: melatonin in the evening advances your clock, while melatonin in the morning delays it. Research using three daily pulses of melatonin showed shifts of up to 1.5 hours per day.
Dosing is between you and your doctor
JetShift advises only on timing. The amount of melatonin you take is a conversation with a healthcare provider, not something an app should decide. We tell you when, never how much.
Partial adaptation for short trips
Full adaptation takes time, roughly one day per time zone for westward travel and 1.5 days per time zone for eastward travel. For a three-day business trip across six time zones, full adaptation is impossible before you have to fly back.
In these cases, JetShift recommends partial adaptation. Shift partway to the destination, enough to improve function during the trip, but not so far that returning home creates a second round of jet lag. This approach prioritizes function over purity.
The same logic applies to shift workers who rotate between day and night schedules. Full adaptation to every shift is often impractical. JetShift can help find the compromise that works for your rotation pattern.
Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin. 2009;4(2):241-255.Caffeine: a tool with limits
Caffeine blocks adenosine receptors, reducing the feeling of sleepiness. It can help you stay alert when your body clock says sleep, but it does not shift your clock. It is a mask, not a cure.
JetShift times caffeine to maximize alertness when you need it and avoids it when it would interfere with the sleep your body actually needs for adaptation. The half-life of caffeine is about five hours, so timing matters.
What JetShift does not claim
JetShift is a personal tool for timing circadian interventions. It is not a medical device. It does not diagnose, treat, cure, or prevent any disease. The science it implements is peer-reviewed, but every body is different. If you have a sleep disorder or medical condition, talk to a doctor.
The recommendations come from published research, but they are general principles applied to your specific schedule. They are not medical advice tailored to your physiology. Use them as a starting point, pay attention to how you feel, and adjust.
Ready to try it?
Enter your next flight and see what the schedule looks like.
Get JetShift