Ever wondered why a bad night’s sleep can leave 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 inside the brain, a cluster 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 gut. When daily routines fall out of sync with this clock, mood can suffer.
Research suggests circadian disruption may contribute to mental health challenges. Our internal clock helps regulate hormones, including melatonin and cortisol. Melatonin helps regulate the sleep-wake cycle.
Levels stay low during the day, rise in the evening as darkness falls (signalling to the brain that it is time to rest), peak in the middle of the night, and drop off in the morning. Cortisol, which helps us respond to stress, typically rises towards morning. These changes prepare the body for activity and rest, while influencing emotional wellbeing.
In a 2024 laboratory study, my colleagues and I investigated whether our internal clocks influence mood independently of everyday activities. We studied 19 healthy adults over 32 hours, measuring mood while minimising changes in their surroundings and behaviour. We found that anxiety-like and depression-like mood were highest at around 8am to 9am, while participants reported greater mental fatigue and lower physical comfort between approximately 6am and 8am.
These patterns were linked to their internal biological clocks, suggesting that mood can fluctuate according to biological time, independently of everyday routines. Circadian disruption has also been linked to depression and bipolar disorder, a condition involving episodes of depression and elevated or irritable mood. The relationship also works in both directions.
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