Sleep and circadian rhythms : what you need to know

Every night, our brain enters an essential rest phase. This recovery time contributes to the overall balance of the body. The circadian rhythm is an internal biological clock that regulates our sleep and wake cycles over a period of approximately 24 hours. This clock controls various biological functions, including falling asleep, waking up, regulating body temperature and releasing certain hormones. 

What role does sleep play in our daily well-being? How does our internal clock, which synchronises our biological rhythms, influence the sleep cycle? What are the consequences of an imbalance in this synchronisation? How can we promote a harmonious sleep-wake rhythm in a natural way? 

Contents 

• What is sleep, and what is meant by “circadian rhythm”? 
• How is the physiology of sleep orchestrated in the human body? 
• Why is the quality of sleep crucial for maintaining balance in the body? 
• What are the signs that suggest a disruption of the biological clock? 
• How can a regular sleep–wake cycle be encouraged naturally? 
• Chronobiology: current areas of research and future perspectives 

1. What is sleep, and what is meant by “circadian rhythm”?

Sleep is defined as a reversible state of rest, characterised by reduced alertness and a reorganisation of brain activity. Sleep is often associated with the night, although it can also occur at other times of the day, such as during naps. 

The circadian rhythm corresponds to a cycle of around 24 hours, controlled by the brain and influenced by the alternation of day and night. It determines the optimal times for falling asleep and waking, and plays a key role in regulating hormone secretion and body temperature. 

The brain, via the suprachiasmatic nuclei, receives light information captured by the retina and adjusts the production of melatonin, a hormone involved in initiating sleep. Exposure to light suppresses melatonin production, while darkness activates it. When the circadian rhythm is disrupted, the body may struggle to maintain a regular sleep–wake cycle, which can lead to sleep problems and affect alertness and mood stability. 

2. How is the physiology of sleep orchestrated in the human body? 

Sleep is coordinated by several internal systems. Exposure to light plays a fundamental role, influencing the suprachiasmatic nuclei in the brain. These regulate melatonin production, which rises as darkness sets in. 

At the same time, fluctuations in body temperature and variations in certain hormones, such as cortisol, help structure the different phases of sleep. This circadian rhythmicity also influences periods of drowsiness during the day. 

However, certain situations can disrupt this natural cycle: 

• Night work can shift the sleep–wake rhythm and alter melatonin production. 
• Significant time-zone changes require a period of adaptation before the internal rhythm resynchronises. 

When such misalignments are repeated, they may lead to persistent fatigue and compromise the quality of night-time rest. 

5. How can we naturally promote a regular sleep-wake cycle? 

Supporting a regular sleep-wake cycle involves making a few simple lifestyle adjustments. Light exposure is essential for synchronising the body’s internal clock. 

Did you know?
Getting natural sunlight in the morning helps establish a coherent rhythm, while reducing artificial light in the evening encourages more natural sleep onset. 

The sleep environment also affects sleep quality. A quiet, dark, and well-ventilated bedroom provides the ideal conditions for rest. Minimising noise and avoiding screens before bed can also help prepare the body for sleep. 

Lifestyle habits directly influence sleep regulation. Regular physical activity supports a balanced biological rhythm, as does a varied diet and eating at consistent times. In the evening, it is best to avoid stimulants and heavy meals that might disrupt falling asleep. 

Daily factors such as meal timing and light exposure help maintain a harmonious sleep-wake cycle. When this rhythm is misaligned, light therapy and chronobiology-inspired approaches may be considered to support better synchronisation of the internal clock. Melatonin, often referred to as the “sleep hormone”, plays a key role in sleep initiation. Naturally produced by the body at nightfall, it signals that it’s time to rest. 

In cases of jet lag, such as after long-haul flights, taking melatonin can help resynchronise the biological clock and reduce jet lag-related fatigue. Melatonin supplementation may be considered for individuals who have difficulty falling asleep, particularly when biological rhythms are disrupted, but it should always complement good sleep habits. 

By adopting these practices, it is possible to restore restorative sleep and a natural rhythm in line with the body’s physiological needs. 

6. Chronobiology: what are the current research areas and future perspectives? 

Chronobiology is a rapidly growing field. Its aim is to understand how the expression of circadian genes adjusts to environmental changes and to explore the underlying molecular mechanisms, particularly those involving the many peripheral clocks present in our organs. 

There are numerous fields of application, including addressing sleep disorders, fatigue, and weight gain - issues often linked to misalignment of the biological rhythm. 

Shift work remains a key area of interest. New studies are examining how irregular schedules affect gene activity and melatonin secretion. Programmes based on chronobiology are being developed to help maintain balance, focusing on adjusting body temperature, light exposure, and meal planning. 

Research and application prospects are wide-ranging. They include improving adaptation to jet lag during travel, enhancing light therapy protocols for phase shifts or recurring sleep issues, and developing personalised approaches (chrononutrition or chronopharmacology). Ultimately, these advancements could lead to more precise tools that respect individual biological differences.