Jet lag disorder occurs when there is a temporary mismatch between sleep-wake cycle timing generated by the person’s internal circadian clock, and the external cues at the new destination.69 (This is commonly known just as “jet lag.”)
In general, eastward travel causes more severe jet lag than westward travel (i.e., moving the clock ahead is harder than moving it back); this is because the biological clock can adjust better when bedtime and waking occur at a later hour. 70
The ability and speed to adapt varies with the individual, however.71
69. Sack RL. Shift work and jet lag. In: Lee-Chiong TL, Sateia MJ, Carskadon MA, eds. Sleep Medicine. Philadelphia, Pa: Hanley and Belfus; 2002:255-263.
70. Drake CL, Wright KP, “Shift Work, Shift-Work Disorder, and Jet Lag,” in In Kryger M, Roth T, Dement W (ed.), Principles and Practice of Sleep Medicine (5th Edition), St. Louis: Elsevier Saunders, 2011, pages 784-798
71. Czeisler CA, Moore-Ede MC, Coleman RM. Rotating shift work schedules that disrupt sleep are improved by applying circadian principles. Science. 1982;217:460-464.