**4.6 Jet lag disorder (JLD)**

JLD results from desynchrony between the external light–dark cycle and an individual's internal clock when travel across time zones occurs more quickly than the endogenous circadian rhythm can adjust. The prevalence of this condition is poorly defined [8]. Factors that affect jet lag severity include the direction of travel and number of time zones traveled, the person's ability to sleep during travel, individual variations in circadian timing, presence of light cues at destination, and intake of alcohol and caffeine.
