Have you ever wondered why a poor night’s sleep can make you irritable, anxious or sad? Poor sleep affects our emotions, but its timing also plays a role.
Our circadian rhythms are nearly 24-hour cycles that regulate alertness, hormones, body temperature and digestion.
Deep in the brain, a group of nerve cells called the suprachiasmatic nucleus (SCN) acts as the body’s master clock. It uses light signals from the eyes to coordinate clocks in other organs, including the brain, liver, heart and intestine.
When daily routines are not in sync with this clock, mood can suffer. Research suggests that circadian disruptions may contribute to mental health problems.
Our internal clock helps regulate hormones, including melatonin and cortisol. Melatonin helps regulate the sleep-wake cycle. Levels stay low during the day, increase in the evening as night falls (signaling the brain that it’s time to rest), peak in the middle of the night, and decrease in the morning. Cortisol, which helps us respond to stress, generally increases around the morning.
These changes prepare the body for activity and rest, while influencing emotional well-being.
In a laboratory study carried out in 2024, my colleagues and I investigated whether our internal clocks influence mood independent of daily activities.
We studied 19 healthy adults for 32 hours, measuring their mood while minimizing changes in their environment and behavior.
We found that anxious and depressed mood were highest between 8:00 and 9:00 a.m., while participants reported greater mental fatigue and lower physical comfort between approximately 6:00 and 8:00 a.m. These patterns were linked to their internal biological clock, suggesting that mood can fluctuate based on biological time, independent of daily routines.
Circadian disruptions have also been linked to depression and bipolar disorder, a condition involving episodes of depression and elevated or irritable mood.
The relationship also works both ways. Mental health issues can disrupt sleep and circadian rhythms, potentially creating a cycle in which poor sleep and psychological distress reinforce each other.
What happens when our clocks become out of sync
Night shift workers may need to stay awake when their body prepares for sleep and rest when their internal clock favors alertness.
This mismatch is known as circadian misalignment.
In a 2020 study, we investigated how this misalignment affected mood in two controlled laboratory experiments involving people with and without shift work experience.
Participants followed schedules designed to simulate day and night shifts, while all other behavioral and environmental factors were controlled, including light exposure, physical activity, food intake and sleep opportunities.
We found that mood and well-being decreased when participants’ sleep and wake times were not aligned with their internal body clock. The effects were observed in non-shift workers as well as shift workers.
These findings suggest that circadian misalignment may influence mood, although larger studies are needed to establish long-term consequences.
Outside of the laboratory, many people experience a time lag called social jet lag. This occurs when sleep and wake times differ significantly between work days and days off.
Artificial light can also affect our internal clocks. Bright light in the evening, including from electronic screens, can suppress melatonin and delay our sleep time.
My research into light sensitivity has highlighted differences between individuals. Some people’s circadian rhythms are much more sensitive to evening light than others, meaning the same exposure can have different effects.
Could changing meal times help?
Light is the most powerful environmental signal to the master clock, but eating also influences the timing of biological processes in other organs.
In a 2022 study, my colleagues and I investigated whether changing meal times could reduce the effects of simulated night work on mood.
Nineteen healthy volunteers followed carefully controlled schedules designed to mimic night shifts. One group ate both day and night, while the other only ate during the day.
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Among participants who ate during both periods, ratings of depressed mood increased by 26.2% and anxious mood by 16.1% compared to their baseline measures.
We observed no significant increases in either measure among those who ate only during the day.
These findings suggest that meal timing may influence how circadian disruptions affect mood. However, the study involved a small group of healthy volunteers under laboratory conditions. Additional research involving shift workers is needed to determine whether eating only during the day can help reduce symptoms of anxiety and depression.
How to support your biological clock
Even though we don’t control our work hours or sleep schedules, certain daily habits can help us keep our internal clocks aligned.
Get the morning light. Light is the most powerful environmental signal to the master clock. Spending time outdoors after waking up can promote alertness and help regulate sleep timing later. The amount of light needed varies from person to person.
Read more: Tired, irritable and craving carbs? Why Darker Days Affect Our Body and Brain
Reduce bright light before bed. Reducing home lighting and limiting screen use in the evening/night can help the body prepare for sleep. The NHS recommends avoiding electronic screens at least an hour before bed.
Keep a regular sleep-wake schedule. Going to bed and waking up at reasonably consistent times can help strengthen circadian rhythms. A systematic review linked regular sleep habits to better health outcomes, including diabetes, heart disease and depression.
Think about when to eat and exercise. Consistent, early meal times can help support the body’s daily rhythms for physical health, although research is needed to establish the best eating times for mental health. Regular physical activity can also be beneficial for sleep. A review of studies found that evening exercise generally did not interfere with sleep, although vigorous exercise ending very shortly before bedtime sometimes did.
Protecting sleep and mental health
Research suggests that the timing of sleep and daily activities can influence mood, although more evidence is needed to determine whether changing these routines can help prevent or treat mental health problems.
Maintaining consistent routines and getting enough sleep can promote emotional well-being, but these habits cannot replace proper treatment. Anyone experiencing persistent sleep or mood problems should see a doctor. Understanding how our internal clocks influence mood could potentially offer new ways to support mental health.
Gn Health
