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.
