Why London-Singapore hits harder than London-New York

Flying east from the UK, towards Asia or Australia, tends to produce worse jet lag than flying the same number of time zones west, towards the Americas, because the human body clock finds it easier to extend a day (staying up later, as on a westward trip) than to compress it (going to bed much earlier, as on an eastward trip). A same-distance westward flight from London to the US East Coast is typically easier to recover from than an eastward flight to a similar time difference in Asia.

NHS advice: start adjusting before you fly

UK health guidance recommends gradually shifting your sleep and meal times by an hour or so per day in the direction of your destination's time zone in the days before a long-haul flight, rather than waiting to adjust after arrival. Staying hydrated and getting daylight exposure at the right times on arrival also speeds up adaptation, more effectively than most over-the-counter remedies.