How our body clock senses light to form circadian rhythm

Humans live in a perpetual 24-hour rhythm. When the morning light falls upon the eye, it travels a path that culminates at the deepest corner of our brain. Known as Hypothalamus, this corner controls vital bodily functions like temperature, hunger, thirst and metabolism. Upon reaching here, the light stimulates brain cells which set off the body's internal clock. As the clock begins to tick, a sense of wakefulness is induced.

It is for this cycle, that we often wake up right before the alarm goes off. Nature has endowed humans with a recurring life rhythm. Known as Circadian rhythms, these are physical, mental, and behavioural changes that follow a 24-hour routine. They work like a music composition by orchestrating a symphony of anatomical processes that shape our daily lives. Researchers have found that animals also exhibit daily patterns that follow a circadian cycle.

Professor Michael Hastings, a circadian neuroscientist, explains, "These rhythms keep everything in time, allowing us to function in harmony with our environment." While their existence has been under study for over 150 years, it is only in recent times that the elements driving these rhythms have started to be unveiled. In this in-depth exploration, we delve into the world of circadian rhythms, uncovering their significance, influence on health, and connection to our metabolism and aging process.

Unraveling how circadian rhythms control the sleep-wake cycle
Dr Ashish Mishra, a cardiologist, working with Wockhardt Hospitals shares, "Circadian rhythms play a central role in regulating the sleep-wake cycle. It senses external light to distinguish when we do feel most awake, and when we feel the sleepiest." The suprachiasmatic nucleus (SCN) in the hypothalamus is the key element that aligns the body's internal clock with the external light-dark cycle. Here's how circadian rhythms spark our sleep-wake cycle:

Light Input: Light is the primary cue that resets the circadian clock. Photoreceptors in the retina of our eyes detect light and send signals to the SCN, informing it about the timing of the external light-dark cycle.

Melatonin Release: The SCN influences the pineal gland to release melatonin, a hormone that helps regulate sleep. Melatonin production increases in the evening as it gets darker, promoting sleepiness, and decreases in the morning when light levels rise, signaling wakefulness.

Cortisol Levels: Cortisol, often called the ‘stress hormone' also follows a circadian pattern. Cortisol levels tend to be higher in the early morning, helping us wake up and feel alert, and decrease throughout the day, reaching their lowest levels in the evening.

Sleep Timing: The SCN's signals influence the timing of our sleep onset and wakefulness. When the SCN receives light cues in the morning, it promotes wakefulness and suppresses melatonin production. In the evening, decreasing light levels allow melatonin to be released, promoting sleep initiation.

Sleep Architecture: Circadian rhythms also influence the structure of our sleep. There are distinct sleep stages, including rapid eye movement (REM) and non-REM sleep. The timing and proportion of these stages are regulated by circadian rhythms, affecting sleep quality and overall restorative effects.

Alertness: These rhythms contribute to daily variations in alertness and cognitive performance. We tend to be most alert and have the best cognitive function during our biological daytime, which varies depending on our individual circadian preferences (Are you a morning person vs. are you a night person?)

Why does our body heat up during the day?
The circadian rhythm regulates body temperature through a complex interplay of physiological processes directed by the hypothalamus. You must have observed how the body temperature tends to rise throughout the morning and peaks in the late afternoon. As the day comes to an end, the temperature gradually decreases, promoting sleepiness.

The body temperature works in sync with the outside temperature. As the light-sensitive cells in the retina receive inputs from external daylight - it signals the brain to heat up the body. This cycle is maintained by the hormones - Estradiol and progesterone, which play a role in regulating body temperature both at the central and peripheral levels. Estradiol tends to facilitate heat dissipation, while progesterone promotes heat conservation and contributes to higher body temperatures.

Dr Narayan Gadkar, circadian expert from Zen Multispecialty Hospital shares, "Disruptions in circadian rhythms, such as shift work or irregular sleep patterns, can lead to temperature dysregulation and impact sleep quality and overall health. Body temperature follows a circadian rhythm, with a peak in the late afternoon or early evening and a trough during the night. This temperature variation can influence sleep quality and cognitive performance."

Can light levels impact our mood?
The mood is significantly influenced by circadian rhythms, as these internal biological clocks play a vital role in regulating the release of neurotransmitters, hormones, and other factors that contribute to emotional well-being.

Circadian rhythms influence the production and release of neurotransmitters such as serotonin and dopamine, which are crucial for mood regulation. Serotonin, often referred to as the "feel-good" neurotransmitter, is synthesised in the brain and its levels are influenced by light exposure. The SCN's synchronisation with daylight helps maintain balanced serotonin levels, contributing to positive mood. Dopamine, associated with motivation and reward, also follows circadian patterns, affecting feelings of pleasure and well-being.

Irregular sleep patterns, such as those seen in shift workers or individuals with irregular schedules, can desynchronise the internal clock and affect neurotransmitter balance. This could potentially lead to mood disorders like depression and anxiety. Inadequate sleep due to circadian misalignment can also contribute to irritability, emotional instability, and decreased overall mood quality.

A dance with destiny: Linking circadian rhythms to health and ageing

Ageing is closely related to changes in circadian rhythms due to the complex interplay between the biological clock and various physiological processes. As people age, there are notable shifts in the regulation and functioning of circadian rhythms, impacting multiple aspects of health and well-being.

Alterations in the sleep-wake cycle are a hallmark of ageing. Older adults often experience changes in sleep patterns, including difficulty falling asleep, fragmented sleep, and early awakenings. These changes are attributed to shifts in circadian timing, reduced melatonin production, and alterations in the sensitivity of the suprachiasmatic nucleus (SCN) to light cues. This phenomenon can lead to "advanced sleep phase syndrome," causing individuals to go to bed and wake up earlier than desired, potentially affecting sleep quality and overall functioning.

Ageing can also lead to changes in the amplitude and synchronisation of circadian rhythms. The robustness of the internal clock weakens with age, resulting in less pronounced daily variations in hormone secretion, body temperature, and other physiological functions. This reduced synchrony between the internal clock and the external environment can contribute to health issues, including disruptions in metabolism, immune function, and cognitive performance.

Digestion and Nutrient Absorption
Circadian rhythms regulate digestion and nutrient absorption by orchestrating the timing of digestive enzyme secretion, stomach acid production, intestinal motility, and hormonal responses. Enzyme activity and stomach acid production peak during the active phase of the day, aligning with meal consumption, while intestinal contractions maintain proper nutrient mixing and absorption.

Hormones like ghrelin and leptin, which regulate hunger and satiety, also follow circadian patterns, influencing meal timing and frequency. This coordination ensures efficient nutrient breakdown and absorption, supporting overall digestive health and nutritional well-being.

The intricate dance of circadian rhythms also shapes our body's destiny. This orchestration extends to regulating the immune system's tempo and influencing gene expression. The disruption of these rhythms can have profound consequences on our health.

Other activities controlled by circadian rhythms are cardiovascular activity, immune system functions, sensory processes and cognitive performance. Circadian rhythms ensure that various bodily functions are synchronised with the external environment, allowing the body to anticipate and adapt to daily changes, optimise energy usage, and maintain homeostasis. Disruptions in these rhythms can lead to health issues and disturbances in sleep, mood, metabolism, and more.

5 important hormones regulated by the circadian rhythm
Various hormones are released in rhythmic patterns that align with the circadian cycle. Here are some of the hormones that follow a day-night pattern:

Growth Hormone (GH): Growth hormone is primarily released during deep sleep and is crucial for growth, cellular repair, and maintenance of muscle and bone tissues. Its secretion peaks during the night, especially in the first few hours of sleep.

Insulin: Insulin, which regulates blood sugar levels, shows circadian fluctuations. Insulin sensitivity tends to be higher in the morning and lower in the evening, contributing to variations in glucose metabolism throughout the day.

Thyroid Hormones: Thyroid hormones, such as thyroxine (T4) and triiodothyronine (T3), play a key role in regulating metabolism. Their secretion follows a circadian rhythm, with the highest levels during the early morning.

Testosterone: Testosterone, an important hormone in both males and females for various functions including reproductive health, muscle growth, and bone density, follows a circadian rhythm with higher levels in the morning.

Harmonising with the rhythms of life: Nurturing circadian well-being

Attaining well-being through a healthy circadian rhythm involves adopting practices that promote the alignment of our body's internal clock with the natural light-dark cycle. Here's how you can attain the optimal circadian rhythm:

Prioritise a consistent sleep schedule. Aim to go to bed and wake up at the same times every day, even on weekends. This consistency helps regulate your circadian rhythm by reinforcing your body's natural sleep-wake cycle. Exposure to natural light during the morning and early afternoon can help reset your internal clock, enhancing alertness during the day and improving sleep quality at night.

Secondly, create a sleep-conducive environment. Make your sleep environment comfortable, dark, and quiet. Limit exposure to screens emitting blue light (phones, tablets, computers) before bedtime, as this type of light can suppress melatonin production and disrupt your circadian rhythm. Engage in relaxing activities before bed, such as reading or practicing mindfulness, to help signal to your body that it's time to wind down.

Lastly, prioritise a balanced lifestyle. Regular exercise can help regulate your circadian rhythm and improve sleep quality. However, avoid vigorous exercise close to bedtime, as it can be stimulating. Be mindful of your diet and meal timing; try to eat at consistent times and avoid heavy meals close to bedtime. Limit caffeine and alcohol intake, especially in the hours leading up to sleep. Engage in stress-reduction techniques like meditation or deep breathing to manage cortisol levels and support your circadian rhythm.