The suprachiasmatic nucleus: the body's master clock

Every day your body runs to a timetable. You tend to feel more alert at some hours and sleepier at others. Body temperature, hormone levels and many other processes rise and fall in a roughly 24-hour pattern. Something has to keep all of that in time.

That something is a tiny region of the brain called the suprachiasmatic nucleus, often shortened to SCN. It is sometimes described as the body's master clock.

What and where is the suprachiasmatic nucleus?

The SCN is a pair of small clusters of nerve cells, around 20,000 in total, in the hypothalamus at the base of the brain. Its name describes its position: it sits just above the optic chiasm, the point where the nerves from the two eyes cross. That location turns out to matter a great deal, because it puts the master clock right next to the wiring from the eyes.

How scientists found the master clock

For much of the twentieth century, scientists knew that animals and people had daily rhythms that carried on even without clocks or daylight, but not where in the body the timekeeper was.

In 1972, two research groups in the United States, working independently, showed in animals that damage to this small area of the hypothalamus abolished daily rhythms in behaviour and hormones. Later experiments went further. In 1990, researchers transplanted SCN tissue between hamsters with different natural cycle lengths, and the animals that received the transplant took on the rhythm of the donor. The clock, it seemed, really did live in the SCN.

The question then was whether the same was true in people. Dr Martin Moore-Ede, a professor at Harvard Medical School from 1975 to 1998 and now Chief Health Advisor to Sensio BioLabs, led the Harvard team that located the suprachiasmatic nucleus in the human brain. That work helped establish the SCN as the master clock in humans, not only in laboratory animals.

How the clock keeps time

Each cell in the SCN contains its own molecular clock: a set of genes and proteins that switch each other on and off in a loop lasting roughly a day. The scientists who worked out how this loop operates, Jeffrey Hall, Michael Rosbash and Michael Young, were awarded the Nobel Prize in Physiology or Medicine in 2017.

On their own, individual cells keep imperfect time. Together, the cells of the SCN keep each other in step and produce a strong, steady rhythm. The SCN then sends timing signals out across the body, influencing sleep and wakefulness, body temperature and hormones such as melatonin and cortisol.

A conductor, not the whole orchestra

Researchers now know that clocks are not only in the brain. Organs such as the liver, heart and muscles have clocks of their own. The SCN acts more like the conductor of an orchestra, keeping all of these local clocks playing in time. When the conductor's signal is weak or mistimed, the players can drift apart.

Why light is the signal that matters most

The SCN runs on a cycle that is close to, but not exactly, 24 hours. To stay in step with the day outside, it needs a daily correction. Light is the strongest of these cues.

The eyes send a direct line to the SCN, separate from the pathway we use to see. At its heart are specialised light-sensitive cells in the retina, discovered around the turn of the century, whose main job is not vision but telling the master clock whether it is day or night. Light at different times of day nudges the clock in different directions, which is why the timing of light matters as much as the amount.

Food, activity and social routines play a part too, but light is the cue the master clock listens to most closely.

What this means for the way we light our lives

For most of human history, the SCN received a clear signal: bright daylight by day, darkness at night. Today many of us spend most of our time indoors under light designed for seeing, not for the body clock, and then spend our evenings under bright artificial light and screens. The signal the master clock receives has become weaker by day and more confused at night.

Understanding the SCN is why lighting designers now talk about the light that reaches the eye, not only the light on the desk, and why the timing of light is part of good design. You can read more in our guide Circadian lighting explained.

Where Sensio BioLabs fits

The discovery of the master clock and of the eye's direct line to it is the science our work is built on. At Sensio BioLabs we study the light people actually receive through the day and how it relates to how they feel and function, working with Dr Moore-Ede and with researchers who know this field best. We will share what we learn here as the work develops.