Fascinating rhythm

Lisa Varadi provides an insight into the body’s circadian rhythms and asks: are you an owl or a lark?

Our world is full of rhythms. The rising and setting of the sun, the waxing and waning of the tides and the changing of the seasons are examples of rhythms, or cycles, within our environment.

They follow a sequence that we can observe and predict.

Not only are cycles present in our external world, but much of our internal physiology follows a rhythmic pattern. In fact, the brain houses a master rhythm, known as the circadian rhythm or circadian clock, that governs the inner processes that follow a daily cycle. Not surprisingly, sleep happens to be one of these processes. Together, sleep and wakefulness combine to form what is known as the sleep-wake cycle. This is the 24-hour period within which we typically spend one-third of the time asleep and two-thirds of the time awake. Due to genetics, there may be some slight variation in the length of a person’s circadian rhythm, but the average length is approximately 24 hours.

The circadian rhythm is regulated in a part of the brain called the suprachiasmatic nucleus (SCN). The SCN detects signals from the eyes regarding the amount of light or darkness. The SCN processes this information and then sends a signal to a gland at the base of the brain called the pineal gland. This gland secretes the sleep hormone melatonin in the presence of darkness. An increase in the secretion of melatonin at night tells the body that it’s time to sleep.

The circadian rhythm is not fully established until the third or fourth month of life. This is why newborns wake frequently. When a child enters puberty, the circadian rhythm tends to shift slightly. This lasts for a few years and is one of the reasons why teenagers are more prone to having later bedtimes and being late risers.

How to bump the early afternoon slump

We’ve all experienced the afternoon sleepiness that often follows lunchtime. Many of us are quick to blame the composition of the meal or our digestive tracts that are using all of the body’s energy to break down food rather than maintain mental clarity. Although eating a carbohydrate-rich meal can make matters worse, the food itself is not the underlying problem. The circadian clock has a natural ‘dip’ about six or seven hours after we wake in the morning. Therefore, it is natural to feel tired around this time. What can be done to offset this slump? Stepping outside for some fresh air and natural light or going for a short walk will help you regain your energy and focus. It’s important to bear in mind that this tiredness usually doesn’t last too long so your alertness and ability to concentrate will soon return.

Owls and larks

In the land of the circadian clock there reside two types of people, the owls and the larks. Most of us fall within one of these two categories but some lie in between. The owls are those who are late to bed and late to rise. The larks are those who awaken early in the morning and tire early in the evening. The terms owl and lark are used to describe what is known as your chronotype. Owls tend to struggle with the typical 9am–5pm work schedule. Waking up in the morning can be fairly challenging as can staying awake during the after-lunch slump. These difficulties often lead to a reliance on alarm clocks and caffeine to keep up with the weekday schedule.

Owls tend to function at their best later in the day. In contrast, larks are able to work harder and more efficiently in the mornings. They may begin to tire towards the end of the workday. Night shifts are a challenge for the lark and socialising and performing hobbies in the evenings are near-impossible pursuits. Larks will often push themselves well into the evening to keep up with the demands of the day.

A person’s chronotype is largely determined by genetics, however lifestyle factors can have an influence. Habits, such as staying up too late, which you have followed over a long period, can impact the timing of your circadian rhythm. The key to survival is a sustainable balance between your daily schedule and your genetic make-up.

Keeping up the beat

Since the circadian rhythm is so vital to our existence, it is essential that it is kept on track. This is the role of what are known as zeitgebers, external cues that help to harmonise our internal clock. There are several environmental factors that help keep the body’s master clock synchronised:

Daylight

Light is the primary zeitgeber because it is the most powerful regulator of the circadian rhythm. In the presence of daylight, melatonin production decreases and we become awake and alert. When daylight is absent, melatonin is secreted, and we become sleepy.

Ambient temperature

A room that is uncomfortable (either too hot or too cold) can shift the circadian rhythm forward and delay the onset of sleep. Frequent waking and less time spent in deep sleep can also occur if the temperature of your sleep environment is not right.

Eating

The times at which we eat can help set our internal clock. Eating smaller meals throughout the day and avoiding heavy or spicy foods before bed can help to make sleep arrive at a proper time. Hunger hormones have a strong influence on the timing of the circadian rhythm. Trying to sleep with an empty stomach can delay sleep onset. Consuming a small nutritious snack before bedtime can offset the wakefulness-promoting effect of hunger.

Physical activity

The timing of physical activity can help to set the circadian rhythm. Exercising at night can increase your body temperature and the secretion of the alertness-enhancing hormone adrenalin, both of which can make falling asleep very challenging. Exercising during the day, on the other hand, can improve your ability to fall asleep and stay asleep at night. Exercise has also been found to reduce the likelihood of developing certain sleep disorders, including insomnia, restless legs syndrome (RLS) and sleep apnoea.

To sleep or not to sleep?

The regulation of sleep is complex. The determination of when to sleep and when to wake is not carried out by the circadian rhythm alone. There is another key participant in the decision-making process. A system known as sleep-wake homeostasis helps to balance sleep and wakefulness. When a person has been awake for a long time, a drive for sleep (or sleep pressure) builds up, which tells the body that it’s time to sleep. Sleep-wake homeostasis ensures that the longer you are awake and the more energy you expend, the sleepier you will be when bedtime approaches.

Extracted from Sleep by Lisa Varadi (£7.99, Quadrille)

Take note!

The National Sleep Foundation’s annual Sleep Awareness Week takes place from 14 to 20 March. Look out for the hashtags #SleepAwarenessWeek and #SAW2021 on social media.

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