All living creatures have cycles of daily activity known as “circadian rhythms.” These rhythms are controlled by an internal time-keeping system, or biological clock, that senses when the day begins and ends.
In plants, biological clocks signal when it’s time to grow. In humans, biological clocks help regulate changes in blood pressure, body temperature, and alertness that vary according to the time of day.
These daily rhythms are based on light – but not on sight. Clearly, a rose is sensitive to light, but it can’t see. And like plants, we humans have special light-sensitive cells called “photoreceptors.”
Photoreceptors send out signals when light strikes them. In mammals, the only known photoreceptors are in the eyes.
Most scientists agree that these light-sensitive cells help set your daily circadian rhythms. But – even though our photoreceptors are in our eyes – you don’t need to have vision to be sensitive to your own circadian rhythms.
Many blind people have normal circadian rhythms. That fact seems to indicate that they have light-sensitive cells – in their eyes – that still are able to regulate their biological clocks.
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