Light Sensitization Study

Sleep disorders affect 35-40% of adults, resulting in diminished quality of life, and
increased morbidity, mortality, and risk of automobile and occupational accidents. Circadian
rhythm sleep disorders form a distinct class of sleep disorders characterized by misalignment
between the timing of sleep and the circadian pacemaker. While light therapy can be an
effective treatment of circadian rhythm sleep disorders, there are numerous practical
limitations.

Light is the most powerful signal from the environment that influences and regulates daily
biological rhythms. It is well-established that the irradiance, duration, and timing of light
exposure all affect the response of the circadian system. While it was once thought that
these responses were mediated through the visual system, it is now known that there is a
network of intrinsically photosensitive retinal ganglion cells (ipRGCs) that mediate
circadian responses to light. Other new studies have demonstrated that recent light exposure
history affects the circadian response to light in humans.

These recent findings have important implications for the use of light to treat circadian
rhythm disorders, including delayed and advanced sleep phase disorders, shift work sleep
disorder, and jet lag, and they may also have relevance for the use of light to treat
seasonal affective disorder. Up to now, little attention has been paid to the duration or
intensity of light exposure prior to such light treatments. The investigators now have
evidence that the human circadian system can become desensitized to light during long
exposures and evidence that it can be sensitized to light by prior exposure to dim light.
These recent findings suggest that light treatment protocols that sensitize the circadian
system prior to the light treatment will be more effective than those currently in use.

The 13-day inpatient studies the investigators propose will examine the effect on the human
circadian system of different durations of dim-light sensitization prior to a standardized
light treatment. These results will be compared within subjects in a randomized cross-over
design study in which each subject will receive a control treatment and a light treatment
with prior dim-light sensitization. The investigators will also include circadian phase
disorder patients to test these mechanisms in the target patient population. Our findings
will provide an important step in understanding how new knowledge about the circadian
photoreceptive system can be used to refine and provide better treatment options for
circadian rhythm disorders.

Inclusion Criteria:

- healthy adults with conventional and regular sleep-wake timing

- non-smokers

- completion of medical, psychological, ophthalmological, and sleep screening tests

- able to spend 13 consecutive days/nights in the laboratory

Exclusion Criteria:

- history of neurological or psychiatric disorder

- history of eye injury, eye surgery, or visual disorder (corrective lenses are
acceptable)

- history of sleep disorder or regular use of sleep-promoting medication

- current prescription, herbal, or over-the-counter medication use (oral contraceptives
are acceptable)