Cabin air is drier than almost anywhere you spend time on the ground
Humidity levels in a pressurized cabin are far lower than typical indoor air, which accelerates dehydration without necessarily feeling like thirst in the usual way — drinking on a schedule rather than waiting to feel thirsty compensates for that.
Caffeine and alcohol both compound the dehydration
Both are mild diuretics, and combined with already-dry cabin air, a drink that would be harmless on the ground has a larger relative effect at altitude — moderating either, especially on longer flights, noticeably improves how you feel on arrival.
Timing meals to the destination, not the departure city
Eating on a schedule closer to your destination's local time, rather than your origin's, gives your body a small head start on the time-zone adjustment that is coming anyway.
Questions people ask
Is it worth bringing your own snacks on a long flight?
Yes, particularly for odd meal timing or dietary needs that onboard service may not match well, and solid food brought from a terminal food area is often more satisfying than snack-sized service portions.
Does drinking more water actually reduce jet lag?
It will not eliminate jet lag on its own, but dehydration makes every symptom of it feel worse, so staying ahead of it removes one compounding factor.