Jet lag is not called "train lag" or "ship lag" for that reason. Only modern jets can transport people rapidly across several time zones, the exact condition that causes jet lag.

When your internal clock is out of sync with the external world, the body feels a "lag." This can be felt as a headache, a stomachache, sleepiness, or insomnia. The science of jet lag is an approximate one, varying with the age of the person, the level of gland secretions, the number and direction of time zones crossed, even the environment of the airplane. We do know the body follows the 24-hour day and night cycle: we're awake when it's light and asleep when it's dark. Internally, we establish a circadian rhythm, regulating temperature and breathing over that 24 hour cycle. In a new time zone, we can reset our internal clock by adjusting meals and light exposure to the new setting.

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Text: Sharon Panelo; Ewen Ku, Editor