Long-distance travel, for business or pleasure, is becoming increasingly common. Any long journey, whether by plane, road or rail, will be associated with "travel fatigue", the combined effects of a changed routine (particularly sleep loss and altered meals) and the general disruption caused by travel. Planning any trip well in advance will minimise many of these problems, but some factors are less easy to guard against; these include sitting in cramped and uncomfortable conditions and, with flights, the hypoxic environment in the cabin. After arrival at a destination in another country, there can be problems with language, altered food and different customs. If the flight has crossed the equator, then there is also likely to be a change in season and natural lighting, and if the flight has, additionally or alternatively, crossed several time zones, then there will also be the problem of "jet lag", caused by a transient dyssynchrony between the "body clock" and the new local time. The new environment might differ from the place of departure with regard to ambient temperature and humidity, altitude, natural lighting (and hence exposure to ultra-violet radiation) and pollution. In all cases, the traveller needs to be aware of these changes before setting off, so that appropriate preparations (different clothing, for example) can be made.