Sleep Length and Circadian Regulation in Humans

Millions of Americans suffer from circadian rhythm sleep disorders, which include shift work
sleep disorder, jet lag, delayed sleep phase syndrome and possibly winter depression. These
conditions are typically characterized by persistent insomnia and/or excessive daytime
sleepiness, impaired performance, and gastrointestinal distress. These negative symptoms
result from a misalignment between the timing of the external social world and the timing of
the internal circadian (body) clock. Circadian rhythm sleep disorders are effectively
treated with bright light, which phase shifts the circadian clock, thereby realigning it
with the timing of the external social world.

It is widely recognized that social influences have led to an increasing prevalence of sleep
restriction in modern society. We recently demonstrated for the first time that short sleep
episodes, when compared to long sleep episodes, markedly reduce phase advances to bright
light. Thus when people cut their sleep short, they inadvertently reduce their circadian
responsiveness to bright light. The mechanism(s) behind these reduced phase shifts to light
are unknown. However, there are at least two aspects of short sleep episodes that could be
responsible for this effect. First, short sleep episodes are associated with partial sleep
deprivation. Second, as humans sleep with their eyes closed and are usually exposed to light
when awake, short sleep episodes are also associated with short dark lengths. Our overall
goal is to determine the biobehavioral mechanisms by which short sleep episodes impair phase
shifts to bright light. Specific Aim 1 is to determine the effect of partial sleep
deprivation on phase advances to light, while controlling for dark length. Specific Aim 2 is
to determine the effect of short dark lengths on phase advances to light while minimizing
sleep deprivation. We will estimate the timing of the human circadian clock by measuring
salivary melatonin, a neuroendocrine hormone released from the pineal gland, and collecting
measures of sleep via actigraphy, and sleepiness, mood, gastrointestinal distress and
cognitive performance via computerized assessment.

Characterization of the separate effects of sleep deprivation and dark length on circadian
phase shifts to light in humans is critical to understanding how humans respond to light
during their daily life activities. Furthermore, the findings of this research will produce
important and practical recommendations for avoiding decrements to phase shifts to light,
thereby optimizing the bright light treatment of circadian rhythm sleep disorders, and thus
improving public health and safety, well-being, mood, cognitive function, and quality of
life.

Inclusion Criteria:

- healthy adult volunteers

Exclusion Criteria:

- color blindness with the Ishihara test

- obese people (BMI > 30)