Crossing time zones costs real, measurable performance, but it's usually a small decrement, sharpest in the first day or two, and it's worse flying east. Full re-adjustment scales with the trip — your clock resets at roughly one zone per day heading east, about two per day west — so a big eastward trip can take a week.
The measured cost is real but smaller than the folklore suggests. Across 20 years of Major League Baseball data, jet lag effects showed up almost entirely after eastward travel, and most of the damage traced back to a single stat: home runs allowed. Across 11,481 NBA games, home teams carrying two hours of eastward lag lost about 4.5 points of scoring margin; the accompanying drop in win rate, around six points, was borderline. Westward lag showed no reliable effect at all. These are margins that decide close games, not ones that turn an athlete into a different player.
Direction matters because the human clock free-runs slightly longer than 24 hours. Stretching your day is easy; compressing it isn’t. Hence the working rule from the 2021 consensus statement: about one time zone per day of adaptation heading east, roughly two per day heading west. Los Angeles to London is a week-long problem. The trip home is three or four days.
Jet lag and travel fatigue aren’t the same thing
Jet lag is the clock mismatch, and it only exists once you cross three or more time zones. Travel fatigue is everything else — the 5 a.m. wake-up, cabin air, immobility, missed meals, and the way it stacks across a season. It happens on a bus ride with zero time zones crossed, and it clears in a day or two with sleep and food. Mixing the two up is how teams end up dosing melatonin for a problem that needed a nap.
What actually has support
Timed light is the strongest lever: morning light after flying east, evening light after flying west, and avoiding it at the wrong end. Low-dose melatonin, 0.5 to 5 mg at destination bedtime, beat placebo in nine of ten trials, mostly for eastward flights crossing five or more zones; larger doses added nothing. Shifting bedtime an hour a day for two or three days pre-departure moves your clock before you board.
For trips under three zones, manage fatigue, not the clock. For long eastward trips, start shifting early and budget a day per zone instead of expecting to arrive ready.