When considering how our modern lifestyles have departed from those of our ancestors (and the problems that arise from these mismatches) the obvious candidates such as diet and daily activities spring to mind, but another significant change that us modern humans are struggling to deal with is the changes/disruptions to our daily cycle, or Circadian Rhythm.
What is a Circadian Rhythm?
A circadian rhythm is a cycle in the physiological processes of living beings (also known as the sleep/wake cycle), plants, animals, fungi and even cyanobacteria all have a circadian rhythm. Our rhythm is as old (if not older) than our species and is hard wired into our biological systems and as such we can’t directly control it (although it can be influenced). Circadian rhythms are controlled by hormones, although they are modulated by external cues such as sunlight and temperature. Circadian rhythms are important in determining the sleeping and feeding patterns of all animals and there are clear patterns of brain wave activity, hormone production, cell regeneration and other biological activities that are all linked to this cycle.
What sets the rhythm?
In humans it’s the circadian clock which controls this rhythm and this is located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain, one in each hemisphere of the brain. The SCN is a tiny pinhead-sized area, containing approximately 20,000 very small neurons, and it has the responsibility for sending signals to several other parts of the brain to regulate the daily sleep-wake cycle, body temperature, hormone production and other functions. The individual neurons that make up the SCN have been found to exhibit a near-24-hour rhythm of activity, suggesting that the clock mechanism actually works on a sub-cellular level. When dissociated from the SCN, the individual cells follow their own intrinsic 24-hour rhythms, but, when incorporated into the SCN, they all fire in synchrony. In experiments on mice where the SCN is completely removed, the mice (which are normally much more active during the night-time and sleep more during the day) show little or no preference for their active time and sleep time, and their activity is sporadic and apparently random throughout the day and night.
The brain’s circadian clock regulates sleeping and feeding patterns, alertness, core body temperature, brain wave activity, hormone production, regulation of glucose and insulin levels, urine production, cell regeneration, and many other biological activities. The most important hormones affected by the circadian clock are melatonin (which is produced in the pineal gland in the brain, and which chemically causes drowsiness and lowers body temperature) and cortisol (produced in the adrenal gland, and used to form glucose or blood sugar and to enable anti-stress and anti-inflammatory functions in the body).
What influences it?
Circadian rhythms are adjusted to the environment by external cues, known as Zeitgebers, the most important of which is daylight. The brain’s internal circadian clock uses these Zeitgebers to naturally synchronise or reset itself each day to within just a few minutes of the Earth’s 24-hour rotation cycle (think of it like a GPS update for your phones clock!). Exposure to natural daylight stimulates a nerve pathway from special photoreceptive ganglion cells in the retina of the eye, cells that are totally separate from the rods and cones our eyes use to generate our everyday image of the world. These cells contain a unique light-sensitive pigment called melanopsin, and are most sensitive to short wavelength “blue light”. Even many blind people can respond to these light-dark cues, as the photoreceptive cells in their eyes can usually recognise daylight, even through closed eyelids. The light-dark signals are sent via the optic nerve to the suprachiasmatic nucleus, which uses them to reset its own circadian clock each day.
The circadian clock does not actually require light to function, and the circadian cycle continues even when individuals are completely cut off from daylight. The light-dark cycle (in concert with other Zeitgebers like meals, ambient temperature, etc), merely acts as an external cue to re-synchronise or entrain the timing of biological rhythms, and to prevent small timing errors from accumulating. Without this important check however, the circadian system can become seriously unbalanced.
What’s your rhythm
Individual circadian periods vary, ranging between 23.5 and 24.5 hours in humans, dependent on variations in the person’s period gene. In humans they have a mean of around 24.2 hours, just slightly more than the Earth’s rotation. About 25% of people have a circadian period which is slightly less than the 24-hour day, and 75% have a circadian period slightly more than 24 hours.
Humans are diurnal animals, naturally active during the daytime, and our circadian rhythms reflect this. Generally speaking, for sleep to occur in the “right” part of the circadian cycle, the time of minimum core body temperature and maximum melatonin concentration should occur towards the end of the sleep period. As a rough guide, core temperature usually reaches its minimum around 4:30-5:00am in the morning in human adults, and melatonin (normally completely absent during daylight hours) typically begins to be produced around 8:00-9:00pm at night and stops around 7:00-8:00am in the morning. The deepest tendency to sleepiness occurs in the middle of the night, around 2:00-3:00am, along with a shorter and shallower period of sleepiness (often referred to as the “post-lunch dip”) about twelve hours later, around 2:00-3:00pm in the afternoon.
Circadian rhythms may be adjusted by up to two hours or so either way according to an individual’s chronotype. Some people tend to wake up early and are most alert during the first part of the day. Others are most alert in the late evening and prefer to go to bed late. By some estimates, as many as 20% of people fall into one of these two categories. In these people, the timing of their circadian period is shifted completely (an effect that is at least partly encoded in their genes), so that morning people wake at a later stage in their circadian day, and are therefore much more alert on waking; evening people, on the other hand, wake too early in their circadian day, and so are less alert and perform poorly in the morning. Typically, this variation is limited to a couple of hours earlier or later than the average; those with extreme body clocks may have difficulty participating in normal work, school or social activities, and are considered to suffer from circadian rhythm sleep disorder.
Development of the Rhythm
Newborn humans (let’s call them babies) spend their first few weeks and months with little concept of night and day. Initially babies do not produce enough melatonin, nor do they produce it consistently at night. This leads to the classic well-known newborn sleep cycle, in which they sleep for very short times and at odd hours. However, babies consistently develop a normal human circadian rhythm (or close to it!) within the first few months of life. Babies begin to develop daily rhythms in temperature, social behaviour, and wakefulness fluctuation early, but sleeping habits are the last to develop. Evening melatonin levels become high enough for the babies to go to sleep at a family bedtime around the 60th day of life. This is also the time when peuple crying and colic usually resolve on their own. Melatonin may be responsible for the end of night-time wailing as well as the beginning of more parent-friendly sleep habits. While babies do not make enough melatonin to go to sleep at night on their own, they have another source, breast milk. Melatonin from the mother is present in her milk. This may help breastfed babies to sleep more soundly. Some doctors and researchers are even suggesting that supplementing melatonin in these early weeks can prevent colic while helping the baby to transition more quickly to a mature circadian rhythm (this is still being researched and I would generally recommend not giving your baby supplements!)
Changes with Age
Changes to the circadian rhythm occur during adolescence and again as we age (>60yrs). Teenagers experience a shift in circadian rhythm that causes them to naturally feel alert later at night, making it difficult for them to fall asleep before 11:00 pm and research suggests this is due to melatonin levels in the blood which naturally rise later at night in teenagers than in most children and adults. The natural shift in a teen's circadian rhythms is called "sleep phase delay." The need to sleep is delayed for about two hours. At first, teens may appear to be suffering from insomnia and hey will have a hard time falling asleep at the usual time. While they begin going to sleep later, they still need an average of nine hours of sleep at night which is often hard as most have to wake up early for school. It is therefore important for them to still go to bed on time as if they go to bed late, they will be unable to get the sleep that they need. This change is a normal part of growing up and with some extra care, teens will quickly adjust to the new sleep schedule of their bodies.
The circadian rhythm also changes later in life, the timing of the circadian rhythm of core body temperature is earlier in both middle-aged and older (>age 60) adults than in young (age 20-30) adults. The circadian phase of melatonin has also been reported to move earlier with age (85-88), as has the timing of the cortisol rhythm. This can lead to awakening earlier in the morning, fewer hours of sleep and less robust body temperature rhythms as we age.
The Circadian rhythm optimises the functioning of an organism by helping to avoid damage to its cells and the accumulation of toxic products but the circadian clock is not resilient to the aging process and its synchronization abilities steadily decline with age. Some researchers have suggested though that it may be possible to resynchronize the circadian clock in an old organism and to reconstitute at least part of the damage-controlling programs and circumvent age-related problems, for example in the brain, which could have huge implications (definitely an area of research to keep an eye on).
What disrupts it?
Your circadian rhythm works best when you have regular sleep habits, like going to bed at night and waking up in the morning around the same times from day to day (including weekends). When things get in the way, like jet lag, small children, daylight savings time, or late night TV, you can easily disrupt your circadian rhythm.
Traveling is a classic circadian disruptor. Changes in time zones, bedtimes and light cues confuse internal clocks, so travelers often arrive at their destination feeling jetlagged.
Other disruptors are artificial lights and electronic devices such as computers, phones and tablet. All of which emit blue light, which trick your brain into thinking it’s daytime and delays the release of melatonin. Even dim lights and small LEDs (like the standby lights) can interfere with a person's circadian rhythm and melatonin secretion.
What happens if it’s out of sync?
Studies suggest a link between exposure to light at night, such as working the night shift, to some types of cancer, diabetes, heart disease, and obesity. That's not proof that night-time light exposure causes these conditions but we do know that exposure to light suppresses the secretion of melatonin, a hormone that influences circadian rhythms, and there's experimental evidence that lower melatonin levels might explain the association with cancer. Studies have also suggested the connection of lower melatonin to diabetes and possibly obesity.
How to honour it
Anyone who’s been camping for anything longer than a weekend will know what I’m talking about, the sense of relaxation that comes from waking up when the sun rises and sitting round the warm glow of a campfire when the sun sets. This is how we lived for hundreds of thousands of years, and it is only recently that this has changed with the advent of gas lamps in the early 1800s and electric light in the late 1800’s. We evolved in a world without artificial lights, governed by the daily cycle of the sun and at a genetic level this is how our bodies still expect to function. Now I’m not suggesting that we should all quit the day job and go and live in a cave but we can make some changes to our daily routines to better honour our genetic ancestry whilst living in today’s world.
1. Maintain a consistent sleep schedule. Sticking to regular bed and wake-up time every day is paramount, even sleeping in late just one morning can throw off your body clock.
2. Allow yourself to wake up naturally in the morning, if you are getting dragged out of deep sleep by an alarm every morning chances are you’re not getting a full sleep cycle, so go to bed earlier.
3. Expose yourself to bright light (natural sunlight is best) when your wake up to help reset your circadian rhythm and deliver an energy boost by raising your body temperature and your cortisol level. Also try to get lots of bright light throughout the day as this will improve your ability to sleep at night, as well as your mood and alertness during daylight.
4. Avoid eating late at night (regular sleep patterns + regular eating patterns = regular rhythm)
5. Adopt a wind-down routine before bed, either;
a. Campfires & Kumbaya or;
b. Avoid looking at bright screens an hour or two before bed and consider wearing blue-blocking glasses or installing an app that filters the blue/green wavelength at night.
6. Consider switching light bulbs. If blue light does have adverse health effects, then environmental concerns, and the quest for energy-efficient lighting, could be at odds with personal health. Compact fluorescent lightbulbs and LED lights are much more energy-efficient than the old-fashioned incandescent lightbulbs but they also tend to produce more blue light. The physics of fluorescent lights can't be changed, but coatings inside the bulbs can be so they produce a warmer, less blue light.
7. Use dim red lights for night lights. Red light has the least power to shift circadian rhythm and suppress melatonin.
8. Ensure good sleep hygiene. Sleep in a dark, cool, well oxygenated room. Think black out blinds, open window (if it’s not too cold!), and eliminate ALL sources of light. A good tip for hiding those pesky little LEDs is to stick little squares of electrical tape over them.
The Takeaway
The circadian clock synchronizes the metabolism and physiology of an organism to enhance fitness and to optimize energy expenditure, so ignore it at your peril! Apply your evolutionary lens and try to better align your modern lifestyle with your ancestral daily cycle. Wake up when the sun comes up, get some exposure to natural sunlight when you wake up, eat and exercise during the day and limit your exposure to blue light in the evening.
