Background: Jet lag is attributed to maladjustment of the body clock after rapid travel across three or more time zones. Because sleep cycles and circadian rhythms are inappropriately phased, jet lag produces a variety of symptoms, including delayed sleep onset, poor performance of physical and mental tasks, fatigue, irritability, headache, gastrointestinal upset, and decreased concentration. Symptom severity increases with the number of time zones crossed and the age of the traveler, and also varies according to direction of travel (eastward journeys cause more severe symptoms than westward).
Pathophysiology: The body clock, located in the suprachiasmatic nuclei at the base of the hypothalamus, controls daily rhythms in temperature, hormone levels, and sleep-wake cycles to promote activity during daytime and sleep and restitution at night. It has a natural periodicity of about 24.5 hours but can be adjusted by zeitgebers (“time-givers”) such as the light-dark cycle, melatonin, and exercise (the weakest of these). The body clock (endogenous control) is usually synchronous with environmental and lifestyle (exogenous) influences in an individual. In time-zone transition, the endogenous and exogenous components are out of synchronization, causing symptoms of jet lag. Symptoms with large endogenous components, such as sleep, mood, and performance, last longer than those that are mostly dependent on exogenous factors, such as food intake and physical activity.
Treatment: Light exposure acts to synchronize the body clock with the sleep-wake cycle. Gradually increasing intensity of light is believed to be more effective than pulses of light or continuous exposure to light of the same intensity. The optimal light wavelength and details of the receptor system are unknown. Changes to the time of sleep shifts the body clock in the opposite direction to that of light exposure, but sleep and exposure to dark are difficult to differentiate.
Exogenous melatonin delays the body clock if taken after waking and advances it if taken in the afternoon or evening. In several studies, 3 to 5 mg of melatonin taken two to three hours before bedtime improved sleep and decreased daytime fatigue. Other studies have shown no benefit from melatonin in shift workers and travelers.
Short-acting hypnotics have provided increased next-day alertness and decreased jet lag in limited studies. Of the stimulating agents, only caffeine is a potentially useful substance in promoting alertness.
Advice for Travelers: For stays of three or fewer days, travelers may feel better if they remain on home time, maintaining normal eating, sleeping, and other activities regardless of local time. For longer journeys eastward, the body clock can be advanced about one hour per day for the three days before departure by exposure to bright light on rising or by taking melatonin in the evening. Melatonin or short-acting hypnotics can also be used to induce sleep at appropriate times for the destination. Bright light, gentle exercise, and caffeine can be used to stimulate daytime alertness on arrival.