Learning to live on a Mars day: fatigue countermeasures during the Phoenix Mars Lander mission

Abstract

Study objectives: To interact with the robotic Phoenix Mars Lander (PML) spacecraft, mission personnel were required to work on a Mars day (24.65 h) for 78 days. This alien schedule presents a challenge to Earth-bound circadian physiology and a potential risk to workplace performance and safety. We evaluated the acceptability, feasibility, and effectiveness of a fatigue management program to facilitate synchronization with the Mars day and alleviate circadian misalignment, sleep loss, and fatigue.

Design: Operational field study.

Setting: PML Science Operations Center.

Participants: Scientific and technical personnel supporting PML mission.

Interventions: Sleep and fatigue education was offered to all support personnel. A subset (n = 19) were offered a short-wavelength (blue) light panel to aid alertness and mitigate/reduce circadian desynchrony. They were assessed using a daily sleep/work diary, continuous wrist actigraphy, and regular performance tests. Subjects also completed 48-h urine collections biweekly for assessment of the circadian 6-sulphatoxymelatonin rhythm.

Measurements and results: Most participants (87%) exhibited a circadian period consistent with adaptation to a Mars day. When synchronized, main sleep duration was 5.98 ± 0.94 h, but fell to 4.91 ± 1.22 h when misaligned (P < 0.001). Self-reported levels of fatigue and sleepiness also significantly increased when work was scheduled at an inappropriate circadian phase (P < 0.001). Prolonged wakefulness (≥ 21 h) was associated with a decline in performance and alertness (P < 0.03 and P < 0.0001, respectively).

Conclusions: The ability of the participants to adapt successfully to the Mars day suggests that future missions should utilize a similar circadian rhythm and fatigue management program to reduce the risk of sleepiness-related errors that jeopardize personnel safety and health during critical missions.

Keywords: Shift work; circadian; light; performance; sleep.

Figure 1

Thirteen subjects' aMT6s data were double plotted, assuming both no adaptation to the Mars day and assuming they did adapt. The assumption that there was no adaptation (A, C) resulted in an arrhythmic pattern for aMT6s with respect to circadian phase, suggesting that the data are not consistent with entrainment to a 24-h day. When the data are plotted according to the Mars day (B, D), a very robust aMT6s circadian rhythm was observed, suggesting that the data are consistent with synchronization to a 24.65-h day. These data support the interpretation that the aMT6s rhythm exhibited a period closer to 24.6 h than 24 h during the study.

Figure 2

(A) Cosinor analysis (gray fitted curve) of aMT6s values (μg/h, filled circles) were used to determine the acrophase (red triangles) for each 48-hour urine collection. White triangles show the acrophase on Earth time collection. (B) Subjects reported work (gray bars) and sleep (black bars) times in a daily diary. This representative subject (#07) worked on a Mars day schedule from May 25 to August 11, 2008. Subsequent acrophases are triple plotted and regression analysis was used to determine the observed period.

Figure 3

(A) aMT6s circadian rhythm data and (B) activity (gray bars) from a single subject (#01) demonstrate the methodology used in analysis of circadian physiology. Data are plotted as in Figure 2.

Figure 4

The daily mean total sleep time for all actigraphy-estimated sleep was 6.2 ± 0.9 hours. Six percent of days had sleep duration > 8 h, and 9% of days had sleep duration of < 4 hours. Forty percent of days had sleep duration of 4 to 6 h, and 45% had sleep duration between 6 and 8 hours.

Figure 5

The duration of sleep during the main sleep episode was dependent on the circadian phase at which sleep was initiated. Actigraph-determined sleep (± SEM) varied significantly by circadian phase (P < 0.001). Sleep initiated outside of the biological night was shorter than when sleep occurred at a normal circadian phase. Only sleep data that had a circadian phase associated with it could be included in these analyses, n = 18 subjects.

Figure 6

Performance was dependent on the number of continuous hours awake. The slowest 10% response times (mean + SEM) significantly increased (A; P = 0.012) and throughput on the ANAM SRT decreased (C; P = 0.024) as a function of time awake. Both performance measures also exhibited evidence of circadian modulation, with a trend for decreased performance during biological night (B and D).

Figure 7

Subjective sleepiness and fatigue ratings were dependent on the number of continuous hours awake and circadian phase. Self-reported sleepiness significantly increased with time awake on both the KSS (A; P < 0.0001) and the ANAM Fatigue scale (C; P = 0.0001). Both measures also exhibited a robust circadian rhythm, with highest sleepiness reported when subjects were awake during the biological night (B, P = 0.0001; and D, P = 0.0003).