Joseph Bass of Northwestern University in Chicago and colleagues have made the surprising finding that flying can sometimes reduce jet lag. After observing daily cycles in blood and tissue oxygen levels in mice kept on a normal light–dark cycle, the team found that if the mice were subjected to a six-hour-long change in light cycle that corresponded to flying east on a jet, those kept in a low concentration of oxygen adapted more quickly. The mechanism involves changing the amount of HIF1α, which is required for oxygen levels to entrain the circadian clock.