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Circadian Entrainment, Sleep-Wake Regulation and Performance During Spaceflight

Principal Investigator:
Charles A. Czeisler, Ph.D., M.D.

Harvard - Brigham and Women's Hospital
Harvard-MIT Division of Health Sciences and Technology

If astronauts participate in exploration missions to Mars, their performance will be critically dependent on their ability to adapt to Mars’ 24.65-hour day. Dr. Charles A. Czeisler hypothesizes that the longer day will cause the body to produce the sleep-promoting hormone melatonin during the waking day, impairing alertness and performance. He is evaluating whether two exposures to evening bright light each day will regulate the release of melatonin, helping the body adapt to the longer days.

NASA Taskbook Entry

Technical Summary

Optimal human performance during space flight requires astronauts to maintain synchrony between the circadian pacemaker, which regulates the timing of sleep, endocrine function, alertness and performance, and the timing of the imposed sleep-wake schedule. Operational demands of space flight necessitate that humans live on day lengths different than the 24-h solar day of Earth (Dijk et al., 2001). Due to orbital mechanics, astronauts are commonly scheduled to the near equivalent of a shorter-than-24-hour day length in Earth orbit on space shuttle missions; moreover, they will be scheduled to the 24.65-h solar day of Mars on the planned exploration class mission to Mars.

Over the past ten years, we have successfully implemented a new technology for shuttle crewmembers involving bright light exposure during the pre launch period to facilitate adaptation of the circadian timing system to the inversions of the sleep wake schedule often required during dual shift missions (Czeisler et al. 1991). However for long duration space station missions it will be necessary to develop effective and attainable countermeasures that can be used chronically to optimize circadian entrainment during extended duration missions.

The purpose of this 65-day long between subjects randomized study is to test three specific hypotheses aimed at evaluating entrainment of the human circadian pacemaker to longer-than-24-hour days.

Specific aim 1: To test the hypothesis that synchronization of the human circadian pacemaker to a sleep-wake and light-dark schedule with an imposed period ~ 4% longer than the pacemakers intrinsic circadian period will be disturbed in men and women.

Specific aim 2: To test the hypothesis that this disturbed circadian synchronization will result in the secretion of the sleep-promoting hormone melatonin during the waking day, disturbed sleep, reduced growth hormone and cortisol secretion, and impaired performance and daytime alertness.

Specific aim 3: To test the hypothesis that two relatively brief (45 minutes) daily exposures to evening bright light (~10,000 lux) will establish a normal entrained circadian phase, in subjects whose imposed sleep-wake and light-dark schedule is ~ 4% longer than their intrinsic circadian period, resulting in improved sleep consolidation, undiminished endogenous growth hormone and cortisol secretion and enhanced daytime alertness and performance as compared to subjects on the same schedule with out the evening bright light exposure.

These hypotheses are based on the results of our preliminary data which indicate that: (a) the period of the human circadian pacemaker after release from entrainment to the 24-hour day is near to but on average slightly longer than-24-hours (Czeisler et al. 1999), (b) the 24.6-h day is outside the range of entrainment of the human circadian pacemaker in the presence of a weak environmental synchronizer (Wright et al., 2001), and (c) intermittent exposure to bright light is a cost effective means of resetting the human circadian pacemaker with respect to power use and astronaut time compared to continuous exposure to light (Rimmer et al., 1999).

During FY02 we completed five experiments. This effort amounts to 325 subject test days in the laboratory during year 2, in addition to the 325 subject days that we conducted in year 1. Originally we proposed to complete 260 subject test days per year and are thus 130 subject test days ahead of schedule. Data collected include: Core body temperature, blood samples (melatonin), Urine samples, Sleep and waking EEG recordings, Subjective sleep quality, Actigraphy, Light intensity, neurobehavioral performance and mood. The successful collection of these data will allow us to test hypotheses 1, 2, and 3 of the project. Data analyses are currently in progress.

The plans for the near future are to continue testing subjects on the Earth and Mars day, analyze data collected, and to test as a countermeasure the ability of brief pulses of bright light to synchronize humans to a dim light-dark cycle for the Earth and Mars day lengths.

This project's funding ended in 2004