Most people run on an internal 24-hour cycle, synchronized to the light and dark cycles of the outside world. Information about external luminescence is conveyed to the suprachiasmatic nucleus (SCN) of the hypothalamus, which incorporates it into what is known as the circadian rhythm, or biological clock.

In cold-blooded vertebrates, deep-brain photoreceptors allow for photoentrainment, the process by which the eyes facilitate setting of the circadian clock. Mammals do not have these receptors; instead, mammalian eyes collect light and send the information back to the SCN through the optic nerve, a pathway called the retinohypothalamic tract (RHT). This is known, in part, because mice with removed eyes cannot reset their clocks.

Oddly enough, about half of all blind people can photoentrain, as can mice that lack functional rods and cones, implying that another receptor capable of processing light exists in the eye. Tracer studies, in which a marker traverses the...

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