Of course the reverse occurs when you go back home. Is it easier on the going away end or the coming home end?
When I flew to Italy, I didn't find the jet lag bad at all, but coming home, I felt exhausted for days. The nice part was that I got to celebrate my birthday twice. :)
“West is best: east is a beast.” — the rhyme of the jet-lagged traveller.
When travelling east, the body takes longer to adjust than travelling west.
In an article in Chaos: An Interdisciplinary Journal of Nonlinear Science, researchers found that our circadian rhythms are slightly longer than 24 hours. This is why travelling west and coping with a later sleep-wake time than at home, comes more easily than travelling east, and hitting the sack earlier.
According to researchers at Oxford University, the reason is the protein, SIK1, which inhibits the impact of light on the brain. Bright light acts as a stimulant but then along comes SIK1 and quietens things down again, limiting our ability to adjust quickly when we cross time zones.
Trying to fit in with a new time zone instantly leads to exhaustion. It’s better to ease into the new zone by consciously manipulating your exposure to light and gradually shifting sleep-wake patterns until you're in sync with wherever you happen to be and using light to assist. Most travellers can push their body clock back by two hours a day.