Whether it’s the physiological monthly cycle of a woman or a plant’s seasonal cycle, the external world influences biology

This column has run weekly for more than two years but, from a biological perspective, that is a bizarre rhythm. Cells and systems in the brain and body have built-in mechanisms to enforce a 24-hour sleep-wake cycle. And light-sensitive cells in the eye and elsewhere keep that synched to the earth’s rotation.

Animals and plants regulate their activities on an annual cycle, becoming frisky in spring and hibernating over the winter. Again, intrinsic mechanisms tend towards an annual cycle and sensors of various kinds nudge it to keep track of the earth’s rotation around the sun.

The physiology of women shows a monthly periodicity in almost all aspects, although it’s not known whether that has more than a coincidental relation to the orbiting of the moon.

But it is culture, particularly religious culture, that has lighted upon the seven-day cycle as an organising structure for our lives. Once something is in the external world, however, it starts to invade our biology, particularly our neurobiology. That’s why it’s so hard to wake up on Sunday morning even if you were silly enough to have set your alarm clock. In the end, science is part of culture.

Photo credit: ‘Animals regulate their activities on an annual cycle, becoming frisky in spring.’ Photograph: Richard Peters/Alamy

This article was first published on the Guardian website as part of the A neuroscientist explains series.