Mainly, this is because it is easier to stay up and go to sleep later than normal, than it is to go to sleep much earlier than you normally would. For example, if you have flown West (say, from London to New York) arriving in the Big Apple at 9pm, you will probably find it easy to go to sleep because your internal body clock is registering the time as 2am.
However, if you were taking the same journey heading East and arrived in London at 11pm, you would find it difficult to sleep as it’s only 6pm in New York!
FLYING LONDON>NEW YORK
Try to get a flight later in the day, ideally any time from 5pm. This means you would arrive in New York from 8pm. Once you’ve cleared security, picked up your bags and made it into the city, you’ll have lost two hours and be feeling exhausted. When you get to wherever you’re staying, you can get straight into bed and hopefully sleep soundly.
This is a much better option than getting a morning flight, which would see you arrive in New York in the early afternoon – or worse – the morning! This would mean staying awake for the entire day until bedtime, which is not good when you’re already tired from travelling.