Circadian Rhythms 148.qxp

RAPID TRANSMERIDIAN TRAVEL CAN PRODUCE JET LAG, WHICH IS CHARACTERIZED BY A CONSTELLATION OF SYMPTOMS, INCLUDING POOR NIGHTTIME SLEEP, INCREASED DAYTIME SLEEPINESS, DECREASED ALERTNESS, IMPAIRED PERFORMANCE, FATIGUE, IRRITABILITY, DEPRESSED MOOD, AND GASTROINTESTINAL DISTRESS.1-5 It is caused by a temporary misalignment between the endogenous circadian clock and the desired destination sleep/wake schedule. Thus, the symptoms of jet lag dissipate as the circadian clock gradually reentrains to the destination time. The duration of jet lag depends on the number of time zones crossed and the direction of travel. Reentrainment is typically slower following eastward than westward flight,2,6 which can be explained by the fact that the free-running period (tau) of humans is slightly longer than 24 hours.7 To understand how this creates directional asymmetry, consider a person with a tau of 24.5 hours, which is the average for free-running blind people.8 If the circadian clock can phase shift by 1 hour per day, then it can assume a period of 23.5 hours (24.5 1.0 = 23.5), equivalent to a phase advance—relative to the 24-hour day—of 0.5 hour per day, or it can assume a period of 25.5 hours (24.5 + 1.0 = 25.5), equivalent to a phase delay of 1.5 hours per day. Thus, given these parameters, phase delays would proceed 3 times as fast as phase advances. Early estimates from actual flights are 92 minutes per day for delays (westward flights) and 57 minutes per day for advances (eastward flights).6 The rate of phase shift for an individual should depend on their tau and on the pattern of light exposure to which they are exposed (see below). Directional asymmetry, with phase delays faster than phase advances, has been shown in more recent carefully controlled laboratory studies of the young9-11 as well as the old.12 In one of these studies of young subjects,10 the sleep/dark episode was shifted 12 hours and bright light was applied to hasten reentrainment. Given a 12-hour shift, circadian rhythms could either advance or delay to reentrain. The direction of phase shift was determined by the timing of the bright light. When the rhythms delayed, they shifted 9.6 hours in 4 days, but when they advanced, they only shifted 6.2 hours. In the study of older subjects,12 the sleep/dark episode was advanced or delayed by 6 hours, and the subjects were kept indoors and, thus, in relatively dim light. After 4 days of an advanced schedule, the rhythms had not yet begun to shift, but after 4 days of the delayed schedule, the rhythms had delayed about 3 hours. Thus, it is much more