The earth has turned on its axis once a day long before intelligent life existed. As a result, life evolved in a rhythmic or cyclical world and adapted to the geophysical cycle created by the earth’s rotation.
Rest-activity cycles are ubiquitous throughout the animal kingdom and they’re controlled by the natural periodicity of the light-dark cycle.
Some animals have adapted to being active in the dark (aka. Nocturnal animals) while others have chosen to be active during the day (aka. Diurnal animals).
Nevertheless, they all show daily patterns of rest and activity.
A daily rhythm of sleep and wake is clearly obvious if we create a continuous recording of brain activity using electroencephalography (EEG) and body temperature in an animal across a number of days.
But before we dig deeper, let’s agree on some terminology:
A rhythm: is a regularly repeated alternation of something (e.g. sunrise and sunset)
The rhythm’s period: is the time between two points on that oscillation such as sequential sunrises (e.g. 24 hours is the period of the natural light-dark cycle)
The rhythm’s phase: any particular point on the cycle (e.g. noon)
So, by recording the daily cycle of sleep, activity, and body temperature of an animal we can identify at what time of the day any particular phase of that cycle occurs.
The circadian rhythm
An experiment was conducted where a squirrel monkey was put under constant dim light conditions. The first few days the animal was sleeping at 2 AM and reaching its lowest body temperature at 9 AM.
During the second week of the experiment, the animal was sleeping at noun and reaching its lowest body temperature at 7 PM.
The animal was running on almost a 26 hours cycle.
Remember, the squirrel monkey is receiving no time cues from the outside world. What we’re seeing is a rhythm that is «internal», and endogenous to the animal. Not imposed by the environment.
Such endogenous rhythms that are close to 24 hours but not exactly 24 hours are called circadian rhythms.
Circa: Latin for about or near.
Diei or dian: Latin for day.
When a circadian rhythm is allowed to be expressed in an environment without time cues, it’s called a free-running circadian rhythm.
When the animal does have access to natural time cues, predominantly sunup and sundown, the circadian rhythm gets readjusted each day to the daily geophysical cycle. And now it’s called an entrained circadian rhythm.
Characteristics of the circadian rhythm
- They free-run with endogenous periods that are about 24 hours
- They can be entrained into the natural geophysical cycle
- Circadian rhythms are temperature-compensated
We tackled the first two characteristics, but what does the last one mean?
Almost all biological processes are temperature-sensitive, they go faster at higher temperatures and slower at low temperatures. But you see, this would not work very well for a clock! It would mess up the circadian clock and start keeping different times at different temperatures.
Many animals like fish and invertebrates attain their core temperature from the environment, and their circadian clock would be really unreliable if it was temperature-sensitive and this is what it means for the circadian rhythm to be temperature compensated, it’s not controlled by neither the body’s temperature nor the environment’s.
Since birds and mammals keep their core temperature rather constant, we don’t have to go through greater details about this feature.
Where is the physical location of the circadian clock?
In the 1970s, scientists were conducting experiments on rats in an attempt to find out the location of the circadian clock. They used radioactive materials and started tracing them.
What they found was that a cluster of neurons with similar functions (medically referred to as a nucleus) located just above the optic chiasm (the intersection between the two optic nerves) was lit up by the radioactive substances. It was called «the suprachiasmatic nuclei» (SCN).
The neural fibers received from the periphery by the SCN are not involved in vision but they convey information about light and dark. These fibers are called «the retinohypothalamic tract» since they begin in the retina (the eye) and end up in the SCN which is a part of the hypothalamus.
The hypothalamus is responsible for regulating many functions of our bodies including metabolism, blood pressure, temperature, water and salt balance, hormones, sleep, etc.
In short, the SCN is the physical location of the circadian clock in the brain.
How is the circadian rhythm entrained?
The process of circadian rhythm entrainment is at the heart of sleep problems that result from jet lag and shift work.
As mentioned before, an important feature of the circadian rhythm is that it can be entrained to the 24-hour geophysical cycle.
The primary stimulus for entrainment is light. And an entraining stimulus is called a « Zeitgeber » which is German for time giver.
Other stimuli such as activity or feeding are considered weak zeitgebers.
The effect of a zeitgeber on a free-running rhythm is described as a phase response curve or PRC for short.
Phase response curve (PRC)
The free-running rhythm of an animal is endogenous, it runs close to 24 hours, it can either be less than 24 hours or more.
Once we establish a baseline for how long this animal’s free-running rhythm is, we expose it to light pulses at different stages through its rhythm.
We then measure the responses to those light pulses as “advances” or “delays” in the free-running rhythm.
An advance in the rhythm means that any of its components such as going to sleep and waking up occurs at an earlier time.
A delay in the rhythm means that any components of the rhythm occurs at a later time.
In our experiment, if we deliver our test light pulses during the day, they have no effect on the free-running rhythm which is only logical. The daylight will be the dominant zeitgeber and our light pulses would be useless at this time.
However, if we emit the light pulses in the first half of the night, they cause the free-running rhythm to be phase-delayed. Any particular phase of the rhythm will occur later than predicted by the free-running rhythm. This makes sense, if you’re about to sleep and then you stand in front of a bright screen, you probably won’t be able to sleep at that point.
Now, if we deliver the light pulses during the second half of the night, the opposite occurs. We would witness a phase advance. This also makes sense, let’s assume you’re asleep and get exposed to bright light at 4 AM in the morning, what’ll happen is that you’d wake up earlier than usual that day and sleep early the next time. So, every phase in your rhythm is advanced.
Let’s apply this information to the real world
Jet lag and the circadian rhythm
Let’s assume you’re flying from London to New York, you arrive in New York at 6 PM which means it’s 11 PM in London, your circadian clock is still on London’s time. So, in New York, you’ll find yourself getting sleepy early (if you sleep at 11 PM in London, you’ll get sleepy at 6 PM in New York) in the evening and waking up earlier in the morning than you’re used to.
But, since you’re seeing light in the evening even when your London-set circadian clock is saying it should be dark. Your circadian rhythm will get phase-delayed and move closer to syncing to New York time. This can take many days to get over.
I hope you’re saying to yourself “Oh, now I understand why he told us about all those experiment shenanigans”
Here’s some helpful advice to get over your jet lag faster
- When traveling several time zones to the west, make sure you see bright outdoor lights in the evening
- When traveling east, make sure you see bright outdoor lights in the morning to maximize your recovery from jet lag
This will be harder to apply when you’re traveling several time zones.
Shift work and the circadian rhythm
Shift work essentially simulates frequent travel across many time zones. This is why it’s an even a bigger problem.
It is divided into
- Day shift: 8 AM – 4 PM
- Swing shift: 4 PM-midnight
- Night (graveyard) shift: midnight – 8 AM
To share the difficulties of working the undesirable swing and graveyard shifts rotations are commonly practiced. They are at one-week intervals and they rotate counterclockwise.
This is the equivalent of traveling 8 time zones to the east each week. And because one week is simply not enough time for the circadian rhythm to get entrained, the worker will struggle with setting his biological clock, which may lead to sleep problems and general health issues.
It is no surprise that many man-made catastrophes happened during swing and night shifts (e.g. Exxon Valdez, Chornobyl, Three Mile Island accidents).
One cannot argue against the importance of shift work especially in fields such as medicine and security. However, we can try to improve our schedule. Here’s how to do it:
- Extend shift rotation to three weeks instead of one week to allow more time for the circadian rhythm to get entrained
- Rotate shifts clockwise
So instead of moving from the night shift to the evening shift and then to the day shift, it would be of great help to do the opposite. Let’s say, for example, you’re working the day shift on Monday, the next best step is for you to work the upcoming evening shift and not the night shift.
This is because your circadian clock can adapt to staying awake a couple more hours easier than suddenly staying up all night.
After you finish your evening shift, the next night shift won’t be as difficult and your body would have had some time to adapt.
Circadian rhythm disorders
Blind people
In a situation where an individual has lost his eyesight, his circadian rhythm is free-running constantly with no ability to be entrained. This may result in difficulties with work schedules and relationships.
Interestingly, people who have lost their vision due to damage to the visual photoreceptors (e.g. the rods and cones) still can entrain to the dark-light cycle but it needs preserving.
The light stimuli that entrain the circadian clock come predominantly from non-visual photoreceptors, these are the retinal ganglion cells, which sit downstream from the primary visual receptors.
However, due to their loss of vision, these individuals are more susceptible to being exposed to ultraviolet light without realizing it burning their retinal ganglion cells in the process and thus, losing their circadian rhythm entrainment.
This is why it’s crucial for these individuals to wear dark sunglasses during the day.
Advanced phase syndrome
This is not a big deal, it just means that a person sleeps early in the evening, and wakes up even more early in the morning. The only downside to this syndrome is that these individuals are not much of a party people.
Delayed phase syndrome
This can be debilitating. These individuals usually cannot fall asleep until 3 or 4 AM in the morning, and as a result, they cannot be fully awake until afternoon.
Imagine this is the case of a college student who has classes each morning or a worker with a typical daytime job.
Seasonal affective disorder (SAD)
SAD is a form of depression that is experienced seasonally. Usually, during the winter time when the sunlight dims and the day length becomes very short.
The consequences of SAD include increased alcohol consumption and suicide rate over the winter season. Symptoms usually improve as spring comes and sunlight becomes brighter.
One method for treating SAD is bright light exposure in the morning; individuals will sit in front of banks of bright light for an hour. And this will result in great symptom improvement.