Disturbed sleep-wake patterns that occur during space missions result in decreased alertness and concentration, compromising the performance and safety of astronauts and NASA ground control workers. Studies show that light treatment can correct similar impairments that occur with shift work, jet lag and sleep disorders. Dr. George C. Brainard is determining the best wavelengths of light to use to readjust biological rhythms and sleep patterns in astronauts and ground control personnel.
Optimizing Light Spectrum for Long-Duration Spaceflight
George C. Brainard, Ph.D.
Jefferson Medical College of Thomas Jefferson University
Ambient light is the primary stimulus for circadian regulation in humans. While on Earth, humans have a 24-hour day/night cycle to maintain healthy circadian entrainment. During spaceflight, astronauts must contend with either a rapidly changing or severely disrupted day/night cycle and must work in a spacecraft interior that is usually dimly lit and with few or no windows. Bright white light has already been implemented as a pre-launch countermeasure on the ground, but has yet to be used during spaceflight.
Providing sufficient white light intensities to work areas in the space shuttle and on the International Space Station (ISS) raises several concerns, among which are heat production and energy consumption. One potentially effective way to mitigate these factors requires a better understanding of how photic input regulates the human circadian system, and an understanding of how to characterize the effectiveness of this input for different wavelengths and wavelength combinations of light. From this data, it may be possible to optimize the light spectrum both as a phase-shifting or entrainment countermeasure for specific circadian disturbances before or during spaceflight and for healthier illumination of spacecraft interiors in general.
This project has three specific aims:
- Test the hypothesis that wavelengths below 440 nm and above 600 nm are active in regulating melatonin secretion.
The action spectrum for human melatonin regulation has previously been established for wavelengths between 440 and 600 nm (Brainard et al., J. Neuroscience, 2001). It is important to extend the action spectrum to wavelengths outside of this range however, especially since astronauts are more likely to be exposed to photic environments in these ranges. For example, astronauts in low-Earth orbit are routinely exposed to an abundance of short wavelength light below 440 nm. Conversely, personnel on any future Mars mission would have to contend with constant exposure to long wavelength light above 600 nm. A key finding is that 420 nm light is indeed effective for melatonin suppression, although not as effective as 460 nm light. Furthermore, the fluence-response curve for 420 nm appears mathematically univariant with the original action spectrum, suggesting the presence of a primary novel photopigment for circadian transduction.
Work on the long wavelength portion of the action spectrum proved technically challenging but is nearing completion. The key finding, as expected, is that long wavelength light even at very high intensities has relatively small impact on circadian regulation.
- Test the hypothesis that there will be a loss of sensitivity to monochromatic light when the eyes are not pharmacologically dilated during the melatonin suppression trials.
It is very common during melatonin suppression studies to dilate the subjects eyes with a pharmacological agent in order to remove changing pupil diameter as a confound to effects of different intensities or types of light. For practical use of these data, it is important to quantify as closely as possible the effects of freely reactive pupils on melatonin suppression. The key finding was that it takes up to 50% more 460 nm monochromatic light to suppress melatonin when the eyes are not pharmacologically dilated.
- Test the hypothesis that there will be a shift in spectral sensitivity of light regulation of melatonin when the eyes are not pharmacologically dilated.
The action spectrum for human melatonin regulation was necessarily established using a pharmacological agent to dilate the subjects eyes. It is unknown, however, whether pupil dilation affected the shape of the resulting action spectrum. In order to test this possibility, the same subjects from Aim 2 were used to establish an action spectrum with freely reactive pupils. Experimental studies for this aim are now complete, and analysis of the research data is currently underway.
Specifically, in the next year of research, we plan to complete a full range fluence-response curve for melatonin suppression with blue enriched prototype lamps. Upon successful completion of scientific trials, these lamps may provide NASA with their first deliverable lighting countermeasure to sleep and circadian disruption for use during spaceflight.
Aside from evidence for a breakdown in physical health, the effects of circadian disruption and sleep loss have long been known to have potentially dangerous behavioral effects. Mental fatigue, diminishment in alertness, loss of psychomotor coordination and physical performance are all commonly found in individuals with sleep loss, sleep debt, or circadian misalignment. The impact of these dangers affects many industries, including transportation, manufacturing, and medicine. It has long been a source of concern for the military as well. Additionally, many people experience the same effects due to jetlag from air travel across several time zones.
Understanding the biological impact of ambient light on the human circadian system is a critical step toward the eventual application of countermeasures to these and other sleep and circadian-related disorders in civilian populations. Quantifying the relative potency of different wavelengths of light for melatonin suppression via the retinohypothalamic tract will provide key insights not only into the basic underlying physiology of this photosensory system, but also produce guidelines for how a misalignment of the system could be most effectively corrected.
This research is also expected to have a wide-ranging impact on future types of artificial indoor lighting and architectural illumination in general. The lighting industry has taken notice of the biological impact different types of light is capable of having, and has already begun to invest in furthering our understanding of lights role for human health. There are a number of existing therapeutic interventions using light that stand to benefit from an enhanced understanding of how specifically different wavelengths of the spectrum affect the human circadian system. A more efficient intervention, with increased potency and/or less side effects, could result.
One such disorder currently being treated with bright white light is Seasonal Affective Disorder (SAD), also known as winter depression. It is estimated that as many as 1 in 5 Americans suffer from SAD or its milder version, subsyndromal Seasonal Affective Disorder (sSAD). Similar bright white light interventions are also used for treating jetlag. Optimizing the light spectrum for specific affective and/or circadian-related disorders could deliver the same biological impact with lower levels of light intensity, and potentially less side-effects.