Daytime Sleep: Natural Rhythm Or Myth?

The circadian rhythm is the body's natural 24-hour clock, which plays a role in various biological processes and systems, including sleep-wake cycles, hormones, digestion, and body temperature. This rhythm is influenced by both internal and external factors, such as light exposure, work schedules, and genetic factors. While most people's circadian rhythms are automatic, disruptions can occur due to shift work, travel across time zones, and health conditions. Maintaining a healthy circadian rhythm involves adhering to a consistent daily routine, regular physical activity, and creating a restful sleep environment. Understanding and aligning with your circadian rhythm is crucial for optimal sleep and overall well-being.

The impact of light on circadian rhythms

Light has a powerful effect on the circadian clock, sleep, and alertness. The body's circadian clock responds to light as a signal to be awake and to darkness as a signal to fall asleep. The circadian clock is most sensitive to light about two hours before bedtime and about one hour after waking up in the morning.

The human circadian system integrates multiple light exposures as short as five minutes, and even intermittent bright light exposure can shift the circadian phase. The phase-advancing effects of daylight have been reported by Roenneberg and colleagues, who, using questionnaire data, found that each additional hour spent outdoors advanced sleep by about 30 minutes.

The availability of artificial light has substantially changed the light environment, especially during the evening and at night. This may increase the risk of developing circadian rhythm sleep-wake disorders (CRSWD), which are often caused by a misalignment of endogenous circadian rhythms and external light-dark cycles. Nocturnal light has been shown to alter circadian rhythms and sleep in humans.

On the other hand, light can also be used as an effective and non-invasive therapeutic option with little to no side effects to improve sleep, mood, and general well-being. Light therapy has been found to be effective in treating seasonal affective disorder (SAD), a subtype of depression characterised by strong seasonal variations in mood states. Light therapy is also effective as a second-line treatment for non-seasonal depression, although it usually takes longer to achieve a therapeutic effect.

Light at the wrong time may disrupt circadian rhythms and sleep, but in the form of light therapy, light exposure can be used as an intervention for psychiatric and other medical conditions.

How to reset your circadian rhythm

Circadian rhythm is your body's natural 24-hour clock that keeps your body operating on a healthy wake-sleep cycle. It is influenced by light and dark, as well as other factors like exercise, eating, and work hours. Here are some ways to reset your circadian rhythm:

• Gradually adjust your sleep schedule by changing your bedtime and wake-up time by an hour or less each day until you reach your desired schedule.
• Get outside early in the day and expose yourself to bright light, especially if you want to wake up earlier. Morning sunlight works well to help reset circadian rhythms.
• Change your meal times—eat earlier if you want to wake up earlier, and eat later if you want to stay up later.
• Limit alcohol and caffeine consumption, especially close to bedtime. Caffeine can keep you awake, and alcohol can disrupt your sleep and amplify the effects of sleep deprivation.
• Avoid exercising too late if you are a morning person, as it may disrupt your sleep. Exercising in the morning can help you wake up earlier and promote healthy sleep.
• Reduce exposure to artificial light, especially blue light, in the hours before bedtime. Blue light is emitted by digital devices like TVs, smartphones, computers, and tablets.

By following these tips, you can help reset your circadian rhythm and improve your sleep-wake cycle.

Circadian rhythm sleep disorders

The circadian rhythm is the body's natural 24-hour internal clock. It is influenced by both internal and external factors, such as light and dark patterns, but exposure to light and darkness has the greatest effect on the circadian rhythm. This rhythm tells the body when to sleep and when to wake up, and it also affects several other body processes, like hormones, digestion, and body temperature.

There are six types of circadian rhythm sleep disorders:

• Delayed sleep-wake phase disorder: This occurs when a person goes to sleep and wakes up much later than normal.
• Advanced sleep-wake phase disorder: This is the opposite of the previous disorder, where a person goes to sleep and wakes up much earlier than normal.
• Irregular sleep-wake rhythm disorder: This disorder results in a chaotic sleep schedule that lacks a clear pattern, with short bouts of sleep spread throughout the day and night.
• Non-24-hour sleep-wake rhythm disorder: This disorder occurs when an individual's circadian rhythm extends slightly beyond the regular 24-hour schedule, causing bedtimes and wake-up times to shift later each day.
• Shift work sleep disorder: This disorder develops in people who work variable or overnight hours, and their circadian rhythm struggles to adjust to the unusual sleep-wake schedule.
• Jet lag disorder: Jet lag occurs when an individual travels across multiple time zones, and their body struggles to adjust to the new time zone, resulting in insomnia, excessive sleepiness, decreased appetite, and mood changes.

The impact of age on circadian rhythms

Circadian rhythms are the body's natural 24-hour clock, which plays a role in processes throughout the body. Circadian rhythms are influenced by factors such as light, and they change as we age.

Circadian Rhythms in Babies, Toddlers, and Children

Newborns typically don't develop a circadian rhythm until they are a few months old, which is why their sleep patterns tend to be erratic in the first few months. A baby's circadian rhythm begins to develop as they experience changes to their bodies and adapt to their new environment. Babies usually start to produce and release melatonin when they are about 3 months old, with cortisol development occurring between 2 and 9 months. Toddlers and children have a fairly regulated sleep schedule once their circadian rhythm and body functions mature, and they need about 9-10 hours of sleep each night.

Circadian Rhythms in Teenagers

During their teenage years, children may experience a sleep phase delay, which is a shift in their circadian rhythm. Their melatonin levels may not rise until around 10:00 or 11:00 pm, and as they are going to bed later, they need to sleep later into the morning. Teenagers still need 9-10 hours of sleep a night.

Circadian Rhythms in Adults

If adults practice healthy habits, they should have a consistent circadian rhythm. If they follow a regular schedule and get 7-9 hours of sleep each night, their bedtime and wake time should remain stable. People over the age of 60 may notice their circadian rhythm changing as they get older, with earlier bedtimes and wake-up times. This is a typical part of aging.

Circadian Rhythms in Older Adults

Circadian rhythms shift with age, gradually becoming earlier in older adulthood. Older adults spend less time in deep sleep, often leading to sleep disruptions and daytime sleepiness. The circadian phase of sleep onset and wakening advances with age, with older adults reporting preferred bedtimes 1-2 hours earlier than younger adults. Older adults experience more awakenings, have longer latencies to fall asleep, and spend less time in deep sleep compared to younger adults.

Mechanisms Underlying Circadian Rhythm Changes with Age

One possible mechanism underlying circadian disruption with aging pertains to the relative synchrony between the master clock in the brain (the suprachiasmatic nucleus, or SCN) and oscillators in other parts of the brain and body. There is evidence from animal studies that the SCN shows reduced responsiveness to light with age, which may contribute to the reduced ability of older adults to adjust to changes in sleep schedules (e.g., due to jet lag or shift work). There is also evidence of age-related changes in the structure and function of the SCN itself, including reduced expression of certain neuropeptides and changes in the electrical activity of SCN neurons.

Health Implications of Circadian Rhythm Changes with Age

Disruptions to circadian rhythms can have both short-term and long-term health consequences. Short-term disruptions may result in delayed wound healing, changes to hormone levels, fluctuations in body temperature, and problems with digestion and bowels. Long-term disruptions can lead to health conditions in multiple body systems, including the cardiovascular and gastrointestinal systems, and may increase the risk of metabolic diseases, chronic inflammation, and cancers.

The impact of travel on circadian rhythms

Travel can have a significant impact on our circadian rhythms, particularly when crossing multiple time zones, as in the case of long-haul flights. This phenomenon, known as jet lag, occurs when our internal circadian clock falls out of sync with the local time at our destination. The human body's internal clock operates on a 24-hour cycle, regulating various physiological and behavioural processes that are crucial for our overall health and well-being.

When we travel across time zones, our body struggles to immediately adapt to the new time zone, resulting in a misalignment of our circadian rhythm. This leads to a range of symptoms, including daytime fatigue, difficulty sleeping at night, reduced appetite, digestive issues, cognitive difficulties, and mood disturbances. The severity and duration of jet lag symptoms depend on the number of time zones crossed and the direction of travel, with eastward travel typically being more challenging for our internal clocks.

To manage and reduce the impact of jet lag, various interventions have been explored, including:

• Melatonin supplementation: Melatonin is a hormone that helps regulate sleep. Taking melatonin supplements close to your target bedtime at your travel destination can aid in re-establishing your circadian rhythms and reducing jet lag symptoms.
• Personalised light exposure: Light is a dominant environmental factor in synchronising circadian rhythms. Exposing yourself to bright light at specific times, such as morning light exposure when travelling west and sunrise light exposure when travelling east, can help adjust your internal clock.
• Chrono-modulation: This approach involves a combination of strategies such as chrono-nutrition (timing meals with circadian rhythms), chrono-exercise (timing physical activity with circadian rhythms), and electroacupuncture, which have been found to improve overall health and resilience during travel.
• Sleep hygiene: Adhering to healthy sleep habits, such as maintaining a regular sleep schedule and creating a restful environment, can improve overall sleep health and mitigate jet lag symptoms.
• Cognitive behavioural therapy (CBT): CBT can effectively address the psychological aspects of jet lag, such as anxiety and stress related to sleep disturbances during travel.

By employing these strategies, travellers can minimise the negative consequences of jet lag and improve their overall adaptation to new time zones, making their travel experiences more enjoyable and less disruptive to their daily lives.

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