EVALUATION OF INTERMITTENT BRIGHT LIGHT EXPOSURE AS
A SPACE FLIGHT COUNTERMEASURE
Charles A. Czeisler, Ph.D., M.D. (Principal Investigator), Claude Gronfier, Ph.D. (Investigator)
Circadian, Neuroendocrine and Sleep Disorders Section, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
INTRODUCTION
During extended duration space station missions, astronauts will be exposed to markedly abnormal light/dark cycles
in terms of both timing and intensity. This exposure is likely to result in a misalignment between the 24-h sleepwake/duty schedules and the endogenous circadian timing system. To prevent sleep disturbances, impaired daytime
alertness, negative mood, and gastrointestinal disorders associated with such misalignment, development of effective
countermeasures to promote circadian entrainment aboard the space station is needed. Recent progress indicates that
three days of exposure to short, intermittent pulses of bright light are sufficient to induce significant phase advances
of the circadian pacemaker, but also that they are more effective on a per minute basis than is continuous bright light
exposure 1.
The current study is aimed to test the hypothesis that a single sequence of intermittent bright light pulses
administered during the early subjective night will induce a significant phase delay of the endogenous circadian
timing system and the physiological variables it controls. We expect that the resetting effect of the intermittent light
exposure will be more efficient than continuous bright light exposure.
CURRENT STATUS OF RESEARCH
Nineteen healthy subjects were exposed to one of the three following conditions designed in our protocol: (1)
continuous bright light exposure (~9,500 lux); (2) continuous very dim light exposure (< 1 lux); (3) intermittent
bright light exposure: six 15-minutes light pulses (~9,500 lux) separated by one hour of very dim light (<1 lux).
The initial melatonin phase was assessed by the midpoint between the DLMOn and the DLMOff 2 during a first
constant routine (CR). The phase-shifting effect of the intervention regimen was assessed using the same procedure
during the second CR following the light stimulus. Subjects remained in dim light (< 3 lux) during the CRs. The
area under the curve (AUC) of the melatonin profile during the 6.5-h exposure was compared to the AUC of the
same period during the CR, as an estimation of the melatonin suppression.
We found that the phase delays observed under the intermittent light condition were comparable to those measured
after continuous bright light exposure, even though the bright light represented only 23% of the 6.5 h exposure.
Indeed, with only 90 minutes of appropriately timed bright light pulses, the circadian timing system was on average
phase delayed by 84% compared to the phase shift obtained with 490 minutes of bright light exposure. Furthermore,
investigating the potential role of melatonin suppression in the phase shifting effect of light, we found that the
melatonin release was suppressed by ~90% during the continuous bright light exposure, whereas it was suppressed
by only ~20% during the intermittent light exposure. Not only these results indicate that the human circadian timing
system may be much more sensitive to light than earlier believed, but also they suggest that the magnitude of the
phase shifts is not a linear function of the degree of melatonin suppression.
FUTURE PLANS
In 2000 and 2001, we will continue to have subjects participate in this protocol, as well as further our ongoing
analyses. Completion of this study will allow refined estimation of the optimal duration of bright light needed for
maximal phase shifting efficiency. This is expected to lead to the development of a practical and efficient
countermeasure to help maintain internal synchrony of the circadian timing system, and thus have a profound effect
on the health, productivity, and safety of astronauts during extended duration space station missions.
INDEX TERMS
Circadian timing system; phase shifts; intermittent bright light exposure; melatonin suppression.
1
Rimmer DW, Boivin DB, Shanahan TL, Kronauer RE, Duffy JF, Czeisler CA. Dynamic resetting of the human
circadian pacemaker by intermittent Bright light. American Journal of Physiology 2000, In Press.
2
Hughes RJ, Sack RL, Lewy AJ. The role of melatonin and circadian phase in age-related sleep-maintenance
insomnia: assessment in a clinical trial of melatonin replacement. Sleep 21:52-68, 1998.