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How the eye signals morning
Special retinal cells, melanopsin intrinsically photosensitive ganglion cells, extend physical wires called axons into the hypothalamus, where the suprachiasmatic nucleus keeps the master circadian clock. Low sun in the sky, with its yellow, orange, and red tones, is the optimal stimulus telling that clock the day is beginning.
How the clock wakes the body
The clock wakes the body through two routes. One runs through peptides and neural signals to the HPA axis, the hypothalamus, pituitary, and adrenals. The other goes down the spinal column and triggers the adrenals to release additional cortisol. Bright light in the first thirty to sixty minutes pushes the peak higher, and without it, stress responses later in the day are bigger and longer.
Cloudy days and windows
Daylight is always out. Overcast skies let through less of the long-wavelength orange and red, but eight AM still looks different from midnight. Light through a window or windshield does not do the job, and a balcony does.
What cells tell the body clock it is morning?
In 2002, David Berson's lab at Brown University reported a new class of light-sensing cells in the retina in a paper in Science. The cells contain a pigment called melanopsin and respond to light even without rods and cones. They make up only a small fraction of the retina's output neurons. Their wires run to the brain's master clock.
How much weaker is indoor light than outdoor?
Typical indoor lighting is about 100 to 500 lux. An overcast day outdoors measures roughly 1,000 to 10,000 lux, and direct sun can top 100,000. Window glass cuts some light, and coatings on modern windows can reflect a large share of infrared. A full-spectrum outdoor scene differs from a lit room in both brightness and color.
How does such a small clump keep time?
The suprachiasmatic nucleus holds only about 20,000 neurons, packed just above where the optic nerves cross. Each of its neurons runs a cycle of clock genes that takes about 24 hours. Coupling between the cells keeps them in sync, so the nucleus acts as one clock. In the early 1970s, lesion studies in rats by Robert Moore and by Friedrich Stephan and Irving Zucker located the clock there.
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