How to trick mother nature into letting you fly around or stay up all night

Abstract

Night shift work and rapid transmeridian travel result in a misalignment between circadian rhythms and the new times for sleep, wake, and work, which has health and safety implications for both the individual involved and the general public. Entrainment to the new sleep/wake schedule requires circadian rhythms to be phase-shifted, but this is often slow or impeded. The authors show superimposed light and melatonin PRCs to explain how to appropriately time these zeitgebers to promote circadian adaptation. They review studies in which bright light and melatonin were administered to try to counteract jet lag or to produce circadian adaptation to night work. They demonstrate how jet lag could be prevented entirely if rhythms are shifted before the flight using their preflight plan and discuss the combination of interventions that they now recommend for night shift workers.

Figure 1

The light PRC was generated from 7 subjects who free-ran through about 3 days (73.5h) of an ultradian LD cycle (2.5 h wake in dim light < 100 lux alternating with 1.5 h sleep in dark) (Eastman and Burgess, unpublished data). Subjects lived on the ultradian schedule on 2 different occasions, once with bright light pulses, about 3500 lux, for 2 h at the same time each day, and once without bright light pulses, counterbalanced. Phase shifts of the midpoint of the melatonin rhythm collected in dim light (<5 lux) before and after the 3 days were plotted against the time of the light pulse relative to each subject's baseline dim light melatonin onset (DLMO) and corrected for the free run when the bright light was not applied. Upward arrow: average baseline DLMO, rectangle: average baseline sleep schedule, triangle: estimated time of body temperature minimum (DLMO+7 h). The solid line is a smoothed curve fit to the 7 points. The melatonin PRC was calculated from the data of Lewy et al. (1998). Subjects (n = 6), living at home, took 0.5 mg melatonin at the same time each day for 4 days. Phase shifts of the DLMO were plotted against the time of melatonin administration relative to each subject's baseline DLMO. A smoothed curve was fit to the data after averaging the 70 data points into 3-h bins.

Figure 2

Preflight schedule to advance circadian rhythms before an eastward flight across 6 time zones. Day –5 shows the sleep schedule and Tmin (body temperature minimum) of a typical young person. On day –4, 0.5 mg melatonin is taken 5 h before baseline bedtime and intermittent bright light pulses are used upon wakening, both timed to phase-advance. Each day the schedule is advanced by 1 h, to keep up with the expected phase advance in the clock, shown by the Tmin, and to ensure that the light and melatonin continue to be administered at optimal times to achieve the maximal phase advance. On the 1st night after the flight, the Tmin would already be advanced to within the sleep period. The flight, as is the case with most flights to Europe from the United States, departs in the afternoon and arrives in the morning. As the clock would already be phase-advanced, all the light exposure upon arrival will occur after the Tmin, thus producing phase advances and completing reentrainment.

Figure 3

Phase shifts in the dim light melatonin onset (DLMO) during 3 days of a preflight schedule similar to Figure 2. For 3.5 h after waking each morning, subjects were either kept in ordinary dim room light (<60 lux) or intermittent bright light (30 min ∼5000 lux alternating with 30 min <60 lux) or continuous bright light (∼5000 lux). Some groups received melatonin or placebo in the afternoon. The 0.5 mg dose of melatonin was administered 5 h before baseline bedtime, timed to coincide with the maximum phase advance of the 0.5 mg melatonin PRC (see Fig. 1). The 3.0 mg was administered 7 h before baseline bedtime because our partial PRC (unpublished data) showed that this is when the maximal phase advances occur with 3.0 mg. Data are from Burgess et al. (2003) and Revell et al. (2005).

Figure 4

Circadian phase during a simulated night shift study in which subjects worked 5 consecutive night shifts (2300 to 0700 h), slept at home in the dark (0830 to 1530 h), and were exposed to various other interventions, such as bright light during the night shifts, to enhance phase delays. The dim light melatonin onset (DLMO) for each subject was determined before (baseline) and after the night shift, and the body temperature minimums (Tmins) (triangles) were estimated by adding 7 h to the DLMO. Earlier subjects (baseline Tmin ≤ 0700 h) showed a range of phase delays depending on the intervention. Later subjects (baseline Tmin > 0700 h) phase-delayed their Tmins into the daytime sleep period regardless of the intervention combination. Data are from Crowley et al. (2003).