Sleep and Circadian Rhythm

How can we optimize sleep? The body operates on a sleep-wake cycle, which is the body’s circadian rhythm or biological clock. You can train this cycle to promote better sleep. We can synchronize sleep to the cycles of light & dark, melatonin production, diet & nutrition, timing of eating, exercise, stress and other factors.

In this blog series, we will give you the tools to reset your circadian rhythm. This will optimize your sleep so keep following us at Medicine with Heart!

We have written an Intro To Sleep blog, then looked at Cognition & Sleep. Today we look at the circadian rhythm of sleep. In future blogs we will cover the best diet for sleep, the best supplements for sleep, how to reset a disturbed circadian rhythm, the importance of optimally aligning diet, light & the natural environment for sleep, and other fascinating information about sleep! 

In this blog, you will learn:

• How the circadian rhythm and sleep are linked
• What are the factors that affect circadian rhythm
• What happens if circadian rhythms are dysregulated

What is the Circadian Rhythm?

The circadian rhythm is the natural cycle of the body. It is based on the body’s biological clock, which follows a roughly 24-hour day. Circadian rhythm determines the timing of being asleep and awake, body temperature, metabolism, physical activity, the release of hormones, digestion, appetite, cognitive function and other body functions (Phana TX, 2019). 

The circadian rhythm is controlled by the suprachiasmatic nucleus (SCN) in the brain. The SCN is a group of 20’000 neurons (nerve cells) that form in the hypothalamus of the brain (NIH, 2023). The SCN regulates circadian rhythms in the body. It is stimulated by exposure to light. The retina of the eye senses light and sends this information to the SCN. In the morning, the SCN gets this signal from the retina and triggers the release of cortisol and other hormones to help the body wake up. When it is dark at night, the SCN sends messages to the pineal gland. This triggers the release of the sleep hormone, melatonin. The SCN controls melatonin release based on how much light the eyes receive. 

Nearly all peripheral tissues in the body have a circadian rhythm. They are regulated and synchronized by the SCN (Phan TX, 2019). The SCN sends output signals to these areas to regulate the timing and peripheral rhythms. For example, the SCN sends signals to the hypothalamus and other areas to regulate sleep and wake cycles, body temperature and hunger and satiety (Phan TX, 2019).

Different factors affect the circadian rhythm. Exposure to light and dark have the biggest influence on circadian rhythm. Other influencing factors are temperature, diet & nutrition, exercise and stress (NIH, 2023). We can modify these factors with the right light exposure, diet, activity, exercise or stress management. 

The Role of Light:

The sleep-wake cycle is most influenced by exposure to light. The retinas of the eyes process light and tell the SCN in the brain whether it is day or night. Sleep quality is related to light exposure during the day. Being in daylight at high intensities is good for sleep. It increases evening fatigue and sleep quality (Blume C, 2019).

The timing of light exposure can shift the circadian rhythm. Natural outside daylight at high intensities can push forward the timing of sleep to earlier hours, affect the duration of sleep, and improve sleep quality (Blume C, 2019). Each additional hour spent outside can advance sleep by about 30 minutes (Blume C, 2019). 

Even intermittent bright light exposure can shift the circadian rhythm. Very brief millisecond flashes of bright light can cause circadian rhythm shifts which are larger than what we see with continuous less-bright light (Blume C, 2019).

Artificial Light 

Natural light, or daylight from the sun, is historically the main type of light influencing the circadian rhythm. But we are now significantly exposed to artificial light indoors.

• Artificial light can delay the timing of the circadian clock and the timing of sleep (Blume C, 2019). Light from LED screens can interfere with sleep and melatonin secretion (Blume C, 2019) . 
• In one study, reading a book from an e-reader for 4 hours before sleep increased sleep onset (time to fall asleep), reduced evening sleepiness, melatonin secretion and even alertness the next morning (Blume C, 2019). This effectively delays the timing of the biological clock (Blume C, 2019). 
• In another example, exposure to artificial light 3 hours before bedtime lead to lighter sleep (Blume C, 2019). 
• Smartphone use before bedtime is linked to more sleeping problems, decreased sleep efficiency, later sleep onset and poor sleep quality. It delays sleep which shortens sleep duration (Blume C, 2019). 

The Role of Temperature:

The circadian rhythm lowers body temperature by about 2 degrees during the night. This cooling down process starts 2 hours before bed, when you start to feel less awake and start thinking of going to sleep. As the body cools down, the pineal gland in the brain releases melatonin. Melatonin release decreases body temperature and prepares for sleep. 

Core body temp drops to its lowest about 2 hours after lights are turned off. In the morning during the last hours of sleep, the body starts to warm up to naturally prepare to wake up. This gives a feeling of alertness for the morning. 

The Role of Melatonin:

Melatonin is a hormone produced by the pineal gland and controlled by the SCN. Melatonin promotes sleep and lowers core body temperature. Melatonin secretion increases soon after darkness. Levels are highest at night and can be 30 times higher than daytime levels (Montaruli, 2021). Melatonin levels fall during the second half of the night (Montaruli, 2021).  

Melatonin production and release are suppressed by light. The circadian rhythm for melatonin is closely synchronized with sleep hours. An imbalance between the circadian clock and light /dark exposure causes mistimed melatonin release and disrupted sleep. 

Melatonin is linked to aging and age-related disease (Montaruli, 2021). The pineal gland that releases melatonin gets calcified with age. Pineal gland calcification and lower melatonin is associated with age-related and neurodegenerative diseases (Montaruli, 2021). 

The Role of Diet:

Diet influences biological and circadian rhythms, body composition and metabolic health (Santos HO, 2022). Diet refers to the quantity, quality and types of food eaten. 

Chrono nutrition, which looks at frequency and timing of meals, is also important for the circadian rhythm. For example, eating during the daytime is optimal for the circadian rhythm, which peaks in the morning and afternoon. Consuming many calories at night can lead to metabolic and cardiovascular issues and disturb sleep (Santos HO, 2022). In addition to timing of meals, aligning the diet with light / dark and the natural environment is important for the circadian rhythm, as is seasonality and what grows locally at what time of year. 

The best diet for sleep, in terms of what to eat when, food sourcing, timing, etc. is a huge topic. We will take a deep dive into diet and sleep in our very next blog, so please follow us for the details.

The Role of Vitamin D:

Sun and light exposure are important to maintain regular circadian rhythm. They also help produce and activate vitamin D. Vitamin D regulates many functions in the body, supports a healthy brain and a strong immune system. Vitamin D receptors are in most cells of the body and UV light from the sun triggers Vitamin D production (Choi JH, 2020). 

Vitamin D is involved in producing melatonin and the regulation of sleep (Romano F, 2020). If vitamin D is low, raising it can increase melatonin levels and help treat melatonin deficiency-related sleep issues (Romano F, 2020).

On average, people usually don’t get enough sun exposure because they are often indoors. This affects sleep due to the bidirectional relationship between vitamin D and sleep duration. Vitamin D levels influence sleep duration and low vitamin D contributes to shorter sleep duration (Prono F, 2022). Vitamin D supplements can improve overall sleep quality (Choi JH, 2020). 

Sunshine, UV-B, Latitude and Vitamin D 

Sunshine rays are made up of Ultraviolet A (UV-A) and Ultraviolet B (UV-B) light. About 95% of the UV rays that reach the ground are UV-A rays while only 5% are UV-B (American Cancer Society, 2019). The UV-B strength depends on latitude. UV radiation is strongest near the equator and weakest at the North and South poles.  

Vitamin D comes from the synthesis of UV-B radiation. Depending on the time of day, time of year and latitude, there may not be enough UV-B to produce vitamin D. So, the farther north you live, the less UV-B light exposure and the less vitamin D is produced from sunlight exposure. Therefore, it is important to consider consciously getting outside to get sun exposure and/ or supplementing with vitamin D. This will maintain good vitamin D levels and optimize sleep duration.