Jet lag happens because your internal body clock, which is tuned to the light and dark of the place you left, stays out of step with local time after fast travel across time zones. This hub explains that clock in plain English: the circadian rhythm, the brain’s master clock, and the hormones and light signals that reset it.
Everything jetlag.info calculates rests on a small number of well-studied biological ideas. You do not need any of them to use the jet lag calculator, but if you want to know why a trip earns the score it does, or why the light planner tells you to seek daylight at one hour and avoid it at another, this is where those answers live. Each page follows the same shape: a short, standalone answer first, then a simple explanation, a deeper dive, a note on what it means for travel, and the real sources behind it.
The ten core topics
| Topic | In one line |
|---|---|
| Circadian rhythm | The roughly 24-hour cycle that drives sleep, hormones and body temperature. |
| Suprachiasmatic nucleus (SCN) | The master clock in the brain that reads light and sets the rhythm. |
| Melatonin | The night-time hormone that signals darkness and marks clock phase. |
| Cortisol | The morning-rising hormone that helps drive daytime alertness. |
| Sleep pressure (Process S) | The build-up of the need to sleep the longer you stay awake. |
| Zeitgeber | External time cues – light, meals, activity – that reset the clock. |
| Light exposure | The strongest reset signal, and how timing changes its effect. |
| Phase response curve | The map of when light advances the clock and when it delays it. |
| Chronotype | Whether you are a lark, an owl, or somewhere in between. |
| Social jet lag | Clock misalignment from schedules, not travel – the same mechanism at home. |
How these pieces fit together
Light reaches the master clock and sets the timing of the whole circadian rhythm. That clock tells the pineal gland when to release melatonin at night and shapes the daily rise of cortisol in the morning. Meanwhile a separate sleep-pressure system builds the longer you are awake. Sleep comes easily when the clock’s “night” and high sleep pressure line up. Cross several time zones and they no longer line up with the local night – that mismatch is jet lag. The fastest lever back into alignment is well-timed light, and the phase response curve is the rulebook for when to seek it and when to avoid it.
New to the topic? Start here
- What is jet lag? – the plain-language overview.
- Why does jet lag happen? – the mechanism, step by step.
- Jet lag symptoms – what to expect and for how long.
- How long does jet lag last? – the “one day per time zone” rule and its limits.
Prefer to skip the theory? Put your route into the jet lag calculator and get a personalised plan.
Sources
- U.S. Centers for Disease Control and Prevention – CDC Yellow Book (Health Information for International Travel), “Jet Lag” chapter.
- NHS – “Jet lag” (nhs.uk/conditions/jet-lag/), last reviewed 31 May 2023.
- National Institute of General Medical Sciences (NIGMS/NIH) – “Circadian Rhythms” fact sheet.
- Nobel Assembly at Karolinska Institutet (2017) – Nobel Prize in Physiology or Medicine awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young for discoveries of molecular mechanisms controlling the circadian rhythm.