History of sleep and circadian

The history of sleep is long and rich. This page provides a brief overview of key developments that have shaped how we now think about sleep and circadian health. By looking back we can see that sleep has had a long rich journey through different eras and cultures, and we can honour the work of our forebears, which has helped us to where we are today.

Sleep has always been important to humans. Some of the earliest documented medical descriptions of sleep date back to ancient Egypt, where sleep medicine was practiced comprehensively even 4000 years ago. They had dedicated sleep temples, described sleep disorders and dream analysis, and even used hypnotics to treat conditions like insomnia. In ancient Greece, Aristotle proposed that a master control organ would shut down to cause sleep. Galen, who came 200 years later proposed that sleep allowed the brain to restore itself, and for digestion to occur, so that the body could recover. Alongside studying physiology, the ancient Greeks saw dreams as having important functions – bringing messages from the gods, and other wisdoms. 

In India, the ancient ayurvedic texts advised that sleep was a basic health requirement that was balanced by diet, mindset and daily routines, and these concepts that are still in use today in modern ayurvedic medicine.

Between the seventh and thirteenth centuries, Arab-Islamic physicians developed detailed ideas about sleep physiology and disorders through more structured research and observations, and even wrote text books on sleep medicine. Al-Rāzī described sleep paralysis; Ibn Sīnā proposed sleep stages based on changes in the pulse, and also described symptoms resembling sleep apnea. Texts also referenced pharmaceutical therapies and herbal remedies for sleep disorders. 

Through medieval Europe sleep was not given as much attention as other biological processes, and in 1662CE sleep was even described as a deficiency in alertness or brain activity by prominent French philosopher, Déscartes. The development of scientific tools such as microscopes helped advance other fields of medicine, but sleep science lagged behind this progress for some time.

The story of circadian science began in 1729, when a French astronomer named Jean-Jacques d’Ortous de Mairan noticed that the leaves of the mimosa plant continued to open and close on a 24-hour rhythm—even in total darkness. This showed that the plant had its own internal clock, or circadian rhythm.  It was an early glimpse of something we now understand more clearly. In humans, we now know that the circadian rhythm is coordinated by a “master clock” in the brain, a tiny structure called the suprachiasmatic nucleus (SCN). This master clock helps our bodies stay in sync with the 24-hour day by aligning everything from sleep to hormone release to body temperature.

Modern sleep science accelerated when electrical brain activity became measurable. In 1929, Hans Berger reported on his use of the human electroencephalogram, or EEG. An EEG is a test that measures the brain’s electrical signals, and in sleep science, EEGs help researchers see how brain activity changes during different stages of sleep.

In 1953, Eugene Aserinsky and Nathaniel Kleitman identified rapid eye movement (REM) sleep, helping overturn the idea that sleep was a single, passive state.

Following on through the 1960s and 1970s the “sleep study” was developed, combining EEG, heart rate measuring, body temperature monitoring and observations. We now call this type of study polysomnography. Today, modern medical instruments have improved our ability to observe and measure what happens during sleep, and have even evolved into personal wearable devices. Much of that progress rests on the shoulders of this prior work. 

The biggest breakthroughs in circadian science are much more recent. Thanks to advances in genetics we have discovered the molecular instructions that keep our body clock running. In the 1980s, American scientists discovered the first “clock” gene in fruit flies, which opened up further genetic exploration to understand how our brain and body keeps a 24 hour rhythm.

In 2017, the Nobel Prize in Physiology or Medicine was awarded to three scientists Hall, Rosbash and Young, who had been working since the 1980s to uncover the biological mechanisms behind the circadian rhythms. Their research showed that clock genes carry instructions for building clock proteins—proteins whose levels rise and fall in a daily cycle. They discovered that a protein called PER accumulates at night and breaks down during the day. These discoveries helped explain how plants, animals, and humans stay synchronized with Earth’s 24-hour cycle.

Another important development came in the 1980s – Borbély proposed a new concept – that two separate but connected hormonal processes regulate when we sleep and wake. This is known as the two-process theory. According to this theory, our sleep-wake patterns are governed by:

• Process C—the Circadian drive for arousal, shaped by our body’s internal clock, repeating roughly every 24 hours
• Process S—the Sleep pressure, the natural buildup of sleepiness the longer we’ve been awake

At its core, Process C keeps us aligned with the rhythms of day and night as various hormones rise and fall to create the right patterns. In parallel, Process S tracks how long we’ve been awake—and how much our body needs rest.

A neurotransmitter is a short-acting chemical messenger that carries signals between nerve cells. A neuropeptide is a long-acting protein molecule that carries messages between nerve cells. 

Research in the 1990s brought forward progress in understanding sleep and circadian rhythms, with the development of a “Sleep Switch” concept by Clifford Saper. This concept helps explain how a complex combination of hormones and neurotransmitters interacts in a balancing act to determine if we are awake or asleep, in alignment with our 24 hour rhythms. Alongside the sleep switch concept was the discovery of an important neuropeptide called orexin (or hypocretin). This special molecule plays a key role in controlling the tipping point of the sleep switch, and more recently has become a target for developing new drug therapies for sleep disorders.

Today, thanks to the work of researchers and physicians that came before us, we understand more than ever how vital our biological clocks are to our health. We are gaining more and more insight into how the complex processes of sleep and wakefulness work in our brains and bodies to create the circadian rhythm. We know that there are genetic factors that determine our own unique 24 hour pattern, we understand how hormones, neurotransmitters and neuropeptides interplay to create the circadian rhythm and trigger sleep to “switch” on or off. We have advanced tools and tests to analyse our biology, and expert clinicians who are deeply trained in sleep and circadian science to help us diagnose and treat health problems.

We understand now that sleep is not something that exists as a separate process. Instead, it is deeply connected and infused into the circadian rhythm, our personal whole-day, whole-body health. 

The circadian rhythm influences nearly every part of life: energy, hunger, metabolism, fertility, mood, and more. And when the clock is disrupted, the impacts ripple out—affecting sleep, blood sugar, and mood, and raising risks for conditions like insomnia, diabetes, and depression. 

The more we learn, the clearer it becomes: understanding and protecting our body’s natural rhythms is essential for protecting our health.