Rods, Cones and Spheres: The Three Ways Your Eyes Detect Light

Most people think our eyes exist for one purpose: to help us see. In reality, our eyes have another equally important job. Besides creating vision, they constantly measure the light around us to help regulate our biological clock, hormones, alertness, mood and sleep.

To do this, the retina contains three different types of light-sensitive cells. Two are responsible for vision: rods and cones. The third type, known scientifically as intrinsically photosensitive retinal ganglion cells (ipRGCs), but which we’ll simply call spheres, doesn't contribute to vision. Instead, it helps our body understand when it is day and when it is night.

Rods: Seeing in the dark

Rods are the most sensitive light receptors in the eye. They allow us to see when light levels are very low, such as at dusk, under moonlight or in a dimly lit room.

Rods are excellent at detecting brightness and movement, but they cannot distinguish colours. That's why the world appears almost black and white at night.

You have around 120 million rods in each eye, making them the most numerous light-detecting cells in the retina. Their job is simple: help you see when there isn't much light.

Cones: Seeing colour and detail

Cones take over when there is plenty of light. They allow us to perceive colours, read text, recognise faces and appreciate fine details.

Humans have three types of cones, each responding most strongly to different parts of the visible spectrum:

• S-cones are most sensitive to shorter (blue) wavelengths.

• M-cones respond primarily to middle (green) wavelengths.

• L-cones respond mainly to longer (red) wavelengths.

Your brain combines information from these three cone types to create the rich colour vision we experience every day.

There are approximately 6 million cones in each eye, concentrated mainly in the centre of the retina where visual acuity is highest.

Spheres: Your body's light sensors

Then there are the spheres. Unlike rods and cones, spheres don't help you recognise faces, read books or distinguish colours. Instead, they measure the overall lighting environment and send that information directly to the brain's master biological clock, located in the suprachiasmatic nucleus (SCN).

You could think of them as your body's built-in light meter.

Their primary task is to let the body know:

• Is it morning or evening?

• Is it bright enough to be daytime?

• Should you stay alert?

• Is it time to get ready for sleep?

Rather than creating vision, spheres help synchronise your body's daily rhythms with the outside world. A well-synchronised circadian rhythm helps you sleep better during the night, have more energy during the day and brighten up your mood.

This is why it’s so important to spend time outdoors during the day so your spheres can do their job.