A Johns Hopkins biologist, in research with
implications for people suffering from seasonal
affective disorder and insomnia, has determined that the
eye uses light to reset the biological clock
through a mechanism separate from the ability to see.
The findings suggest that patients with trouble
sleeping or with seasonal depression — disorders
that can be linked to lack of exposure to daylight —
could benefit from development of easier, more
available tests to determine if they are able to detect
light properly for functions distinct from
normal sight, said Samer Hattar, assistant professor of biology in the
university's Zanvyl Krieger
School of Arts and Sciences.
"It seems that even if individuals have normal sight,
they might be having a malfunction that is
contributing to their inability to detect light, which can
adversely affect their biological clocks,"
Hattar said.
Writing April 23 in the Advance Online issue of Nature
(available at www.nature.com) and in the
May 1 print issue, Hattar and colleagues report that they
genetically modified mice so that a
particular set of retinal ganglion cells — cells that
receive input from the rods and cones of the animals'
eyes and send information to the brain — no longer
functioned.
The mice were still able to use light to see normally
but had great difficulty synchronizing their
circadian rhythms to light/dark cycles, the constant
lengthening or shortening of daylight hours that
occurs depending on the time of year.
Prior research in the field leads the researchers to
believe that because the rodents' internal,
biological "clocks" are out of sync with the solar day, the
rodents would have difficulty learning and
sleeping on a regular, 24-hour cycle. The team has not yet
tested that hypothesis.
"This research illustrates that there are two distinct
pathways for the two different aspects
of light detection: image-forming and non-image-forming,"
Hattar said.
The team's next step will be working toward a broad
understanding of the functions of light for
animals, and differentiating between those that are
associated with image formation and those that
are associated with simple light detection.
Even without that additional research, however, Hattar
and his team are convinced, on the basis
of a long line of work by other researchers, that daily
exposure to natural light enhances memory,
mood and learning.
"Our tips are simple: Get out in the sun for at least
a little while each day," Hattar said.
"There's a reason why we seek the sun and the beach and we
feel better when we can sit in the sun
and bask.
"Also, avoid very bright lights during the night, as
exposure to them can cause a malfunction in
your biological clock," he said. "The idea is to keep your
internal rhythm in sync with the cycle of the
sun: exposure during the day, when the sun is out, less
exposure at night, when the sun is down, so to
speak. I am convinced that this will help improve your
memory and your learning."
Ali D. Guler, Jennifer L. Ecker, Cara M. Altimus and
Haiqing Zhao, all of the Department of
Biology at Johns Hopkins, are co-authors. Other authors are
Gurprit S. Lall, Alun R. Barnard and
Robert Lucas, all of the University of Manchester, United
Kingdom; Shafiqul Haq, Hsi-Wen Liao, Hugh
Cahill, Tudor C. Badea and King-Wai Yau, all of the Johns
Hopkins School of Medicine; Mark Hankins,
of the University of Oxford, United Kingdom; and David M.
Berson, of Brown University.
This research was supported by the National Institutes
of Health, Biotechnology and Biological
Sciences Research Council, David and Lucile Packard
Foundation and Alfred P. Sloan Foundation.
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