The World Health Organization is recommending that people who fly for longer than four hours get up and move around especially those who take sleeping pills...so their circulation keeps, well, circulating. They're saying that the risk of developing fatal blood clots (or deep vein thrombosis) increases if people are immobile in cramped conditions. Consuming alcohol should also be avoided. Shorter people are also less likely to get blood clots as their legs dangle as opposed to taller people.
Other factors that can affect a person's chances of developing blood clots are obesity, genetic conditions, age, use of oral contraceptives, and being shorter than 5-foot-4 or taller than 6-foot-4. The theory is that short people are less mobile because their feet dangle and taller people because they are more cramped