Jet lag is your internal clock refusing to keep pace with the sun. As a rule of thumb the body drifts about one hour per day eastward and an hour and a half per day westward, so six zones east needs around six days while six zones west clears in about four. Enter zones and direction for an estimate, plus the light strategy that shortens it.
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Scientific references
Deep in the brain the suprachiasmatic nucleus keeps a roughly twenty four hour rhythm, calibrated daily by morning light. Fly across zones faster than that clock can chase the sun and every timed process, sleep, appetite, alertness, digestion, lands in the wrong local hour. The mismatch is jet lag, and its severity scales with zones crossed.
Why East Is Harder
The internal day of most people runs slightly longer than twenty four hours, so lengthening the day by flying west fits the clock’s natural drift, while shortening it by flying east fights it. That asymmetry produces the classic rule, adaptation runs near one hour per day eastward but one and a half westward. Six zones east therefore costs about six days, the same west about four.
Light Is the Lever
Light exposure shifts the clock, but timing flips its effect. After eastward travel, seek bright morning light at the destination and avoid late evening light, which pushes the wrong way. After westward travel, do the reverse, chasing evening light and shading early mornings. Getting this backwards, easily done, actively extends the lag.
Melatonin as the Second Lever
Half to three milligrams of melatonin taken at the destination’s evening, timed a few hours before the target bedtime for eastward trips, is the best supported pharmacological aid, with meta analyses showing meaningful benefit for multi zone crossings. Sedative sleeping pills shorten sleep latency but do not shift the clock itself.
Short trips of a day or two often go better keeping home time, sleeping and eating on the origin schedule, rather than half adapting and re-lagging on return.