It's an experience that any seasoned traveller knows all too well, getting off a long-haul flight with that feeling of not quite knowing if you should be awake or asleep and knowing that the next couple of days are going to be rough. When we travel across time zones our bodies have to adapt to a set of circumstances that we just weren't designed to cope with. Deep within our brains is a tiny but crucial structure known as the hypothalamus that regulates, among many other things, our circadian rhythms otherwise known as the sleep/wake cycle. When we skip across time zones on a long flight our hypothalamus has to play catch up with the change in environment. What many people had noticed with the problems associated with this disruption to our body clock's normal functioning was that the severity of the jet lag was significantly affected by the direction of travel. For most people the feelings of tiredness and the general sense of dislocation were much worse when travelling from west to east rather than the other way round. If your journey takes you from east to west, say flying from London to Los Angeles, your brain and body experience what is known as phase delay. This means that as the clocks go backward your body has to set itself back rather than leap forward. In practice this is likely to mean you will simply have to stay awake for a few extra hours in order to adjust to your new setting. Travelling west to east on the other hand means you are skipping forward with the clocks, known as phase advance. In effect this means it feels like you're missing a much bigger chunk of time and maybe even a whole night's sleep. So now you know!