A person with depression might sleep much longer than usual and still wake feeling exhausted. Later that evening, when they want to go to bed, they may find themselves wide awake. Researchers are investigating whether patterns like these could help explain why antidepressants work better for some people than others.
Author
- Timothy Hearn
Lecturer, University of Cambridge; Anglia Ruskin University
The clue may lie in the body’s internal clock , which helps regulate when someone sleeps, wakes and feels alert.
An Australian study examined 11,380 people, predominantly with severe or recurring depression. About 23% met the researchers’ criteria for a proposed subtype called “circadian depression”, named after the body’s roughly 24-hour internal clock.
Participants needed at least three of six features, including being a night owl, sleeping longer during depression, morning tiredness and seasonal changes in mood or behaviour. The other features involved habitually sleeping and waking later, and having different sleep schedules on working days and days off.
Compared with other participants, this group reported less benefit and more side-effects from two common types of antidepressants, SSRIs and SNRIs, which affect how brain cells communicate through chemical messengers. However, some people with these sleep patterns still reported benefiting from these medicines.
The researchers also found genetic associations with later sleep timing, reduced sensitivity to insulin, the hormone that helps regulate blood sugar, and a protein involved in inflammation, the immune system’s response to injury or infection. These associations reflect inherited tendencies, rather than evidence that participants actually had blood-sugar problems or inflammation.
The findings are preliminary . Participants recalled their symptoms and medication experiences, and those in the proposed group generally had more severe depression. The researchers did not measure their internal clocks or establish cause and effect.
Some features overlap with recognised forms of depression , so it remains unclear whether this is a distinct subtype. All participants had European genetic ancestry, limiting how widely the findings can be applied. The 23% figure therefore cannot be assumed to represent everyone with depression.
How the body clock affects mood
The body clock coordinates roughly 24-hour cycles , known as circadian rhythms, which affect sleepiness, body temperature and hormones, the chemical messengers that help regulate the body. Light reaching the eyes helps keep this clock aligned with day and night. Meals and physical activity also provide timing cues.
When internal timing clashes with daily demands , sleep can suffer. Someone whose clock runs late may feel alert at midnight but have to get up for work while their body is still expecting sleep. Being a night owl is not necessarily a problem. Persistent disruption is the concern.
Sleep and mental health can reinforce one another . Disrupted sleep may worsen depression, while depression and stress can disturb sleep and daily routines.
One possible link involves brain systems that regulate emotion and reward , including the ability to enjoy pleasurable experiences. Animal experiments suggest that disrupting the body clock can affect these systems and produce depression-like behaviour.
In small laboratory experiments , researchers altered the sleep schedules of shift workers and people who did not work shifts. When their sleep and waking times were out of step with their internal clocks, participants reported worse mood. They had not been recruited because they had depression, and temporary low mood is different from clinical depression.
Conventional antidepressants might leave a timing problem untreated, but that remains a hypothesis. The Australian study did not test body-clock treatments, and its findings cannot establish whether someone with disrupted rhythms would benefit more from an alternative medication.
Could light therapy help?
Light therapy involves sitting near a specially designed bright-light box at a scheduled time, often in the morning. Morning light can shift a late-running body clock earlier , helping someone feel sleepy and wake earlier.
The treatment is used for seasonal affective disorder, depression that follows a seasonal pattern, often during darker months. Evidence suggests it can also help with depression that has no seasonal pattern.
A review of 11 clinical trials involving 858 people examined bright-light therapy for non-seasonal depression, including bipolar depression. Bipolar disorder involves depression alongside episodes of unusually elevated or irritable mood and increased energy.
In trials reporting remission, about 41% of people receiving bright light improved enough to fall below the threshold for depression, compared with 24% receiving dim light or treatment without bright light.
But these trials did not select people with the proposed circadian subtype, so they cannot tell us whether that group benefits more. Light may also influence mood through brain pathways separate from the body clock .
Timing and medical guidance matter. Light can shift the clock earlier or later depending on when it is used. People with bipolar disorder need particular supervision because light therapy can sometimes trigger unusually elevated or irritable mood.
Regular waking times, daytime activity and meals may also support daily rhythms . Melatonin, a hormone that rises in the evening and signals biological night, can help shift a delayed clock when carefully timed. However, studies have not established melatonin supplements as an effective depression treatment.
Towards more personalised treatment
Could measuring someone’s internal clock help doctors choose treatments? Wrist-worn activity monitors can estimate sleep patterns, while saliva samples taken throughout an evening can show when melatonin starts rising, offering a more direct measure of internal timing.
Researchers need to establish whether the proposed group really has distinct biological rhythms, then test whether choosing treatment based on those measurements helps people recover sooner.
For now, sleep patterns cannot reliably predict which antidepressant will work best, and the findings provide no reason to change prescribed medication without medical advice.
If future trials confirm a benefit, a prescription for depression might one day specify when to receive light as carefully as which medicine to take.
