People who struggle to sleep at night, wake up feeling tired despite spending enough time in bed, or find themselves sleeping longer when their mood worsens may be experiencing more than ordinary sleep problems.

Researchers say disruptions in the body's internal clock may be linked to a form of depression associated with more severe symptoms.

Known as circadian depression, the condition describes a pattern of depression linked to disturbances in the body's natural sleep-wake cycle.

The internal clock helps regulate when people feel sleepy or alert and also influences body temperature, energy levels and other biological processes.

The findings were published in the British Journal of Psychiatry. Researchers analysed clinical and genetic data from 11,380 people with depression who took part in the Genetics of Depression Study. Nearly one in four participants met the proposed criteria for circadian depression.

The researchers found that people who showed this pattern generally experienced more severe depressive symptoms and had a higher genetic risk of other health problems, including additional mental health conditions, inflammation, metabolic disorders and cardiovascular diseases.

“The findings suggest that the timing and quality of sleep may be important when assessing depression. However, researchers have not established whether disruptions to the body's internal clock directly cause depression or occur alongside it,” they said.

Depression can affect people in different ways, but common symptoms include persistent sadness, loss of interest in activities, low energy, difficulty concentrating, changes in appetite, and feelings of hopelessness. Some people may sleep too little, while others may sleep for long periods but still feel tired.

In people who experience circadian-related sleep problems, other signs may include regularly going to bed and waking up at irregular times, struggling to fall asleep at night, feeling unusually sleepy during the day and finding it difficult to feel alert in the morning.

These symptoms do not automatically mean someone has circadian depression. Sleep problems can have many causes, and a health professional needs to assess a person's symptoms, their duration and how they affect daily life.

The body's internal clock follows an approximately 24-hour cycle and is strongly influenced by light and darkness. Light entering the eyes sends signals to the brain, helping it determine when the body should be awake and when it should prepare for sleep.

Bright light during the day, particularly in the morning, helps keep this clock on schedule. Exposure to bright light late in the evening can delay the body's natural sleep timing, making it harder for some people to fall asleep.

Researchers say several factors may contribute to disruptions in this rhythm. These include irregular sleep schedules, insufficient exposure to daylight, excessive light at night, genetics, environmental conditions and physical health.

People who naturally stay awake late, commonly known as night owls, may be particularly affected when their preferred sleeping hours conflict with work, school or other daily responsibilities. This can lead to insufficient sleep and persistent tiredness.

Emiliana Tonini, a researcher at the University of Sydney who studies body-clock rhythms and mood disorders, said that bright light during the day, especially in the morning, helps stabilise the body's internal clock, while light exposure at night can delay it.

The researchers said these disruptions may cause different biological rhythms to fall out of sync, potentially affecting both mental and physical health.

The study also raises questions about why some people respond less effectively to conventional depression treatment. Previous research involving participants in the Australian Genetics of Depression Study found that antidepressants appeared to work less well for people who naturally stayed awake late at night. They also reported more side effects.

Researchers say treatments that target the body's internal clock, including light therapy and melatonin in appropriate cases, could help some patients. Light therapy involves exposure to bright light at specific times, while melatonin is a hormone involved in regulating sleep timing.

However, more research is needed before these approaches can be recommended specifically for circadian depression. The researchers have also called for studies involving more diverse populations to determine whether the findings apply to people from different backgrounds.

For now, maintaining regular sleep and waking times, getting daylight during the day and reducing bright light exposure late at night may help support a healthy sleep routine.

People experiencing persistent sadness, loss of interest, severe tiredness or ongoing sleep difficulties should seek professional assessment rather than assuming the problem is caused by their body clock.

Researchers stress that people naturally have different sleep patterns, and being a night owl does not mean a person has depression. The important concern is whether changes in sleep and mood persist and begin to interfere with everyday life.

Although circadian depression is still an emerging area of research rather than a universally established diagnostic category, understanding the relationship between sleep and mood could help clinicians identify different patterns of depression and develop more personalised treatment approaches.

The World Health Organisation (WHO) estimates that 322 million people worldwide were living with depression in 2023, affecting about 5.2 per cent of adults.

In Kenya, an analysis of the 2022 Kenya Demographic and Health Survey found that 2.6 per cent of participants reported having depression. However, the actual burden may be higher because some people remain undiagnosed.