A 2024 laboratory study of 19 healthy adults found that anxiety-like and depression-like mood were highest at around 8am to 9am — a pattern linked to the body’s internal biological clocks rather than by daily routines, according to research by Sarah Chellappa of the University of Southampton published in The Conversation.
Poor sleep doesn’t just leave you tired. It can leave you irritable, anxious or sad, and the timing of those feelings matters too. Circadian rhythms — near 24-hour cycles that regulate alertness, hormones, body temperature and digestion — shape emotional wellbeing, and when daily routines fall out of sync with them, mood can suffer.
The brain’s master clock sets the emotional tone
Deep inside the brain, a cluster of nerve cells called the suprachiasmatic nucleus acts as the body’s master clock. It relies on light signals from the eyes to synchronize clocks in other organs, such as the brain, liver, heart and gut.
The clock also helps regulate hormones that influence mood. Melatonin levels stay low during the day, rise in the evening as darkness falls, peak in the middle of the night and drop off in the morning. Cortisol, which helps the body respond to stress, typically rises toward morning. Research suggests circadian disruption may contribute to mental health challenges, and it has been linked to depression and bipolar disorder.
Mood bottomed out in the early morning, the 2024 study found
Chellappa and her colleagues studied 19 healthy adults over 32 hours in a laboratory, measuring mood while minimizing changes in their surroundings and behavior. Mood ratings for anxiety-like and depression-like states peaked at roughly 8am to 9am, whereas participants noted increased mental fatigue and reduced physical comfort from about 6am to 8am.
These patterns were linked to the participants’ internal biological clocks, suggesting mood can fluctuate according to biological time, independently of everyday routines.
Shift work and social jetlag throw the clock off
Night shift workers may need to stay awake when their bodies are preparing for sleep, a mismatch called circadian misalignment. In a 2020 study, Chellappa’s team ran two controlled laboratory experiments with people with and without shift-work experience, simulating day and night shifts while controlling light exposure, physical activity, food intake and sleep opportunities. Mood and wellbeing reduced when sleeping and waking schedules were misaligned with the internal body clock — in non-shift workers as well as shift workers.
Outside the laboratory, many people experience “social jetlag,” when sleep and waking times differ substantially between working days and days off. Bright evening light, including light from electronic screens, can suppress melatonin and delay sleep. Chellappa’s research on light sensitivity has also found that some people’s circadian rhythms are much more sensitive to evening light than others, so the same exposure can have different effects.
Eating at night worsened mood in a 2022 night-shift simulation
Light is the strongest signal for the master clock, but eating also influences the timing of biological processes in other organs. In a 2022 study, 19 healthy volunteers followed schedules mimicking night shifts. One group ate during both day and night; the other ate only during daytime.
For participants who ate during both periods, depression-like mood ratings rose by 26.2% and anxiety-like mood ratings by 16.1% relative to their baseline measurements. The daytime-only eating group did not observe significant increases in either measure. Because the research was conducted with a small group of healthy volunteers in a laboratory setting, additional studies with shift workers are required to determine if daytime-only eating can help alleviate symptoms of anxiety and depression.
Four daily habits that keep the clock aligned
We may not control our working hours, but some habits can help. Getting morning daylight is the most powerful environmental signal for the master clock. Reducing bright light before bed — the NHS recommends avoiding electronic screens for at least an hour before bedtime — may help the body prepare for sleep. Maintaining a fairly regular sleep-wake schedule may strengthen circadian rhythms; a systematic review linked consistent sleep patterns to improved health outcomes, such as diabetes, heart diseases and depression. Regular and earlier meal times could help sustain the body’s daily rhythms for physical health, though more research is required to determine optimal eating schedules for mental health. Consistent exercise may also improve sleep, and a review of studies indicated that evening workouts typically did not disrupt sleep, though intense exercise finishing very near bedtime occasionally did.
Your questions about the body clock and mood
What time of day is mood lowest? During the 2024 laboratory study, anxiety-like and depression-like mood peaked at roughly 8am to 9am, whereas participants experienced heightened mental fatigue and diminished physical comfort from about 6am to 8am. Both patterns were linked to their internal biological clocks.
Can eating only during the day protect mood during night work? In the 2022 simulation, participants who ate during both the day and night experienced increases in depression-like mood of 26.2% and anxiety-like mood of 16.1% compared with baseline, while those who ate only during the daytime did not observe significant increases in either measure. Further research involving shift workers is needed to establish whether this can help reduce symptoms of anxiety and depression.
Does evening screen light really affect sleep? Bright light in the evening, including light from electronic screens, can suppress melatonin and delay sleep time. The NHS recommends avoiding electronic screens for at least an hour before bedtime.
The connection goes in both directions: mental health challenges can disturb sleep and circadian rhythms, possibly forming a cycle where poor sleep quality and psychological distress strengthen each other.
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