Explained: Sleep, circadian rhythms and mental health

The timing of our sleep is regulated by circadian rhythms – 24-hour cycles that are part of the body’s internal clock, co-ordinating different processes to occur within us at different times of the day and night. These rhythms influence every aspect of our biology, including our moods, alertness, body temperature, and cardiovascular function.  

Sleep and circadian rhythm disturbances (SCRD) have been linked to multiple categories of mental disorder. They can even predict the onset of mental disorders – like anxiety, depression, or psychosis – and serve as the earliest signs of relapse. Furthermore, treating sleep disturbances can alleviate the symptoms of mental disorders, and vice versa.  

For example, emerging evidence suggests that treating a sleep disorder like insomnia using Cognitive Behavioural Therapy (CBT) may also treat co-occurring mental disorders, potentially even preventing them from surfacing in the first place. Similarly, intervening in circadian rhythm disorders with light therapy may simultaneously treat associated mood disorders. 

While progress has been made in understanding the links between sleep, circadian rhythm, and mental health, the biology binding sleep and circadian rhythm disorders to the causes and symptoms of mental disorders, and vice versa, remains unclear.  We’re also not sure how or why treatments targeting sleep and circadian rhythm disruption work to improve mental health disorders. 

Sleep is a reversible state of reduced awareness and consciousness, during which we cycle through two different modes: REM – or rapid eye movement – sleep and non-REM – NREM – sleep. REM and NREM feature different patterns of brain activity and are thought to serve different functions, including memory processing and emotional regulation. However, precisely how the mechanisms and functions of REM and NREM interact is not known. 

By measuring brain, eye, and muscle activity patterns during sleep, researchers have started to develop a deeper insight into what happens in the brain during sleep cycles – including in people who experience sleep disorders and associated mental health problems. 

For example, some studies found that REM sleep occurred earlier in the sleep episode in people with depression than in healthy people, and with increased duration and density. Subsequent studies found that earlier REM sleep was not specific to depression, but could also occur in related mental health disorders, such as anxiety and psychosis. Others highlight the links between disrupted brain waves during NREM sleep and some symptoms of schizophrenia. 

Common sleep disorders include insomnia, trouble falling and/or staying asleep, sleep apnea, a sleep-related breathing disorder, and parasomnias, such as night terrors and sleepwalking. 

Every biological system within the body – from cardiovascular and digestive systems, to sleep-wake cycles – follow circadian rhythms that are connected to a ‘circadian pacemaker’. This is located in the suprachiasmatic nucleus (SCN) in a part of the brain called the hypothalamus.  

The SCN sends signals to regulate activity throughout the body. It is highly sensitive to light, one external signal it uses to coordinate the body’s internal processes.  

When our circadian rhythms are fully aligned with the 24-hour day-night cycle, they promote consistent, good quality sleep that restores the body. However, when this rhythm is thrown out of synch, it can cause significant sleep disturbances.  

Among the most common examples of circadian rhythm disruption is jet lag. This occurs when circadian rhythms are disrupted by long haul travel, usually over several time zones. It can cause mood disturbances, anxiety, tiredness, a change in appetite, and affect stomach and bowel function. 

Another is shift work disorder, which is associated with symptoms that are similar to jet lag – and with increased likelihood of mental health problems like depression. This is caused by frequently rotating shifts or regularly working at night.

The world around us has an impact on our sleep and circadian rhythms. Evidence suggests this then has a knock-on effect on our mental health.  

Environmental disruptors such as air, light, and noise pollution can have a negative effect on the quality and duration of our sleep, as well as the timing of our internal body clock.  

Built-up, urban areas are full of these stressors – as well as additional societal pressures – and have been associated with an increased risk of mental disorders in European, US, and Asian populations. 

A 2020 World Health Organization and European Environment Agency briefing on health risks caused by environmental noise concluded that more than 100 million people were exposed to harmful levels, particularly of road traffic noise, in urban areas. Of these, 22 million suffer chronic ‘high annoyance’ and 6.5 million suffer chronic sleep disturbance. We know that continual loud noise can lead to chronic sleep disturbances – while daytime noise increases stress levels that can impact our circadian rhythms. 

Meanwhile, more than 80% of the global population is impacted by light at night. A cross-sectional study undertaken in the United States found that light at night was positively associated with mood and anxiety disorders, bipolar disorder, and depression in adolescents.  

Being exposed to light at night delays the ‘circadian pacemaker’ . This disruption can be made worse by a lack of exposure to sunlight during the daytime, and additional exposure at night time to blue light from our computer and phone screens. 

While great progress has been made in identifying correlations between sleep and circadian rhythm disorders, and mental disorders, our understanding of how sleep relates to the causes and symptoms of mental illness remains superficial, as does our knowledge of how mental health problems influence sleep and circadian rhythms.  

To increase our knowledge on this, and improve the quality of evidence available to us, we need to address the following issues: 

• most studies focus on adults rather than adolescents – the age when most mental disorders manifest 

• majority of sleep studies have been performed in laboratory settings, which limits the ability to study changes in sleep over time, in the normal sleep environment

• we need to better understand how behaviours during the day, such as napping and low exposure to light, affect our sleep and mental health

• we need to identify common measures for sleep and circadian function as well as mental health to enable comparison across studies

• most individuals are studied in the chronic phase of related illness

• most research is conducted in high income countries, and fails to account for societal, familial, cultural, economic, and geographical diversity 

• lived experience experts from diverse backgrounds are needed to ensure research fully encapsulates their important perspectives. 

Wellcome’s new funding call, ‘Integrating sleep and circadian science into our understanding and treatment of mental health problems’, aims to give researchers across a range of disciplines the support and time they need to make breakthroughs in the field of sleep, circadian rhythms and mental health.  

We hope the more detailed knowledge of the relationships between sleep, circadian rhythms, and mental health that comes from this award will enable us to find new ways to detect mental health disorders and intervene in them as early as possible.