to the alterations of external cues (i.e., reduced light/dark modulation) and possibly delayed bedtimes, as well as the fact that the intrinsic period of the circadian pacemaker is longer than 24 hours (Gundel et al., 1997). Monk et al. (1998), however, analyzed the circadian rhythms of four astronauts (using body temperature) prior to, during, and following a 17-day shuttle mission. From this study, the authors determined that circadian rhythms in orbit appear to be very similar in phase and amplitude to those on the ground.
Far fewer analyses have been conducted on circadian rhythms over long-duration missions. A case study involving an astronaut on a mission to Mir (Monk et al., 2001), revealed that a 24-hour circadian rhythm was maintained for about the first 3 months, with disruptions in sleep and a reduced circadian amplitude occurring during the last 12 days (Mallis et al., 2004).
Another case study that was conducted over a 438-day Mir mission revealed delays in circadian rhythms (Mallis and DeRoshia, 2005). This and other circadian delays are attributed to a variety of factors including: the alterations of external cues, i.e., reduced light/dark modulation (Mallis and DeRoshia, 1995); possibly delayed bedtimes; as well as the fact that the intrinsic period of the circadian pacemaker is longer than 24 hours (Gundel et al., 1997. These inconsistencies in circadian desynchronization may also be due to individual differences, as some individuals (as previously mentioned) are more susceptible to sleep loss or the debilitating effects of shifted work-rest cycles (Dinges, 2004; Mallis and DeRoshia, 2005).
Factors that Contribute to Circadian Desynchronization During Space Flight
Lighting remains the most significant external cue for altering the phase of the circadian rhythm. Lighting is so effective, in fact, that numerous Category I and Category II ground-based laboratory studies have shown that timed exposure to specific types of bright light and blue-enriched (short-wavelength) light serves as an effective countermeasure for circadian phase-shift and performance deficits due to sleep deprivation (Czeisler et al., 1986; Brainard et al., 1988; Czeisler et al., 1989; Brainard et al., 2001; Czeisler et al., 1995; Lockley et al., 2003; Brainard and Hanifin, 2005; Cajochen et al., 2005; Gronfier et al., 2007; Lockley, 2007).
Any natural lighting to which crews are exposed on a spacecraft may impact their circadian adaptation. Note that the ISS and docked shuttle orbit the Earth every 1.5 hours, resulting in 16 sunrises and sunsets every 24 hours, causing the natural lighting cues surrounding the ISS to vary greatly from the terrestrial 24-hour day and night cycle. Indeed, astronauts on shuttle and ISS are no longer exposed to the natural 24-hour day and night cycle of the Earth but, rather, rely on cues from artificial lighting in addition to those from any of the sunrises/sunsets. Thus, the astronauts' circadian rhythms may be altered by these changes in light exposure.
Less-than-optimal artificial lighting conditions have been reported on the ISS (Category IV). Station lighting is provided by both incandescent and fluorescent light sources. Over time, this lighting has degraded due to lamp burnout and the difficulty in supplying replacement lamps on orbit. Over the 9 years of ISS construction, lamps were resupplied piecemeal, with one or two lamps being shipped up by the Soyuz. The resultant decline in on-board lighting eventually was addressed by the first major resupply by STS-114 in July 2005. As soon as the lamps were delivered to the ISS, however, the re-lamping duty was officially given a relatively low priority. Crew members raised this priority significantly, however, because of their desire to improve the illumination on board the station (see Appendix 1 for additional details). This was not only to avoid eyestrain but because, as artificial lighting can impact circadian rhythms and acute alertness, inadequate lighting contributes to circadian desynchronization and fatigue.
Slam shifting, which is an acute shift in the sleep/wake schedule to accommodate a docking or critical task in flight (Leveton et al., 2006), is another risk factor for circadian desynchronization in the current space flight
