Our natural inclination as humans is to sleep during the night and be awake during the day, and this is due to the effect of our circadian rhythm. It’s what creates your regular sleep schedule.
In this article we’ll discuss how the circadian rhythm works, how your internal biological clock helps you sleep, and how by mistiming sleep you could develop a circadian rhythm sleep disorder, the most common of which being delayed sleep phase syndrome.
What is a Circadian Rhythm?
Circadian Rhythms are daily fluctuations in bodily processes that occur in a cycle of approximately 24 hours, based upon the daily rotation of the earth around the sun.
The term comes from the Latin word “circa”, meaning around, and “dies”, meaning day. So its literal meaning is “around a day”.
Circadian rhythms are not only found in humans. Virtually all life forms including plants, fruit flies, fish and bacteria, all have circadian rhythms.
The processes that are involved with sleep naturally operate to a circadian rhythm. We are designed as humans to naturally follow a sleep wake cycle that is in alignment with the day night cycle, to sleep during the night, and be awake during the day.
How Your Circadian Rhythm Helps You Sleep
How sleepy we feel, also known as our sleep debt, is determined by two factors operating in two different parts of your brain:
- The length of time you have been awake
- Where you are in our circadian rhythm
The first factor is sort of obvious. The longer we stay awake, the more sleepy you tend to be. But have you ever noticed that sometimes, despite being awake for so long, that you just can’t fall asleep? Or that sometimes you feel over-tired and you actually start to feel more alert again? That’s because of your circadian rhythm.
If you try and fall asleep when you haven’t been awake for long or if your circadian rhythm is telling you that you should be awake, you’ll have a hard time sleeping. But if you’ve been awake for a good amount of time and you’re at a point in your circadian rhythm where your body is starting to wind down, you’ll be off like a light.
That’s why if you want to sleep better it’s so important that you satisfy those two factors.
Your Body Clock – How your body knows the time
In the 1970s, two groups of researchers found a part of the brain called the suprachiasmatic nucleus (SCN) located in the hypothalamus region of the brain, situated just behind the eyes. The SCN contains two clusters of cells that act like clocks. The SCN is your own internal body clock.
Zeitgeber – Time Giver
The body clock calibrates itself using a number of time cues. These cues are called zeitgebers, a German word meaning time giver.
The most important zeitgeber is light. Sunlight travels into your eyes, through the optic nerve, which leads to the SNC. The SNC contains special photogenic cells that are able to detect the level of light.
When there’s light your body thinks the sun’s up so it must be daytime and so it responds by keeping you awake. When there’s no light your body assumes the sun must have gone down so it must be night-time and so it begins to make you feel sleepy.
To do this your body uses a hormone called melatonin to make you feel sleepy. When there’s no light melatonin is produced by the pineal gland. Melatonin is known as the hormone of darkness for this reason. But when light enters your eyes, the pineal gland stops producing melatonin. This is how the body naturally aligns the sleep wake cycle to the day night cycle.
Other zeitgebers are temperature and the regular timing of social activities, exercise, and meal times. The regular timing of these events help keep your sleep schedule on track.
But the Length of Your Circadian Rhythm isn’t 24 hours
So if your body clock calibrates itself mainly to light, what would happen to your sleep timing if there was no light?
In the summer of 1972, French cave explorer Michel Siffre lived in a Texas cave for 7 months. With no watch and no daylight, his sleep was monitored by a surface team to assess how Michel’s sleep wake cycle would change with no outside cues to the time of day.
The results were that Michel’s day was 25 hours long, advancing an hour day. However there was one important flaw to this experiment. While Michel had no exposure to daylight, he was still exposed to electric lighting, and this is what is said to have artificially extended his sleep wake cycle.
The experiment however got scientists interested, and it was later found that with no cues to the time of day, the body’s sleep wake cycle is 24.2 hours, remarkably close to the 24 hour day night cycle.
How it Compensates – Entrainment to the Day Night Cycle
This slight drift in time isn’t an issue, because the body clock naturally calibrates itself using these time cues, so your circadian rhythm keeps to 24 hours.
The technical term for when the body clock calibrates itself to these external cues is “entrainment”. Researchers at Harvard have shown that humans can be entrained to a slightly longer cycle (24.65 hours) and a slightly shorter cycle (23.5 hours). The longer cycle of 24.65 hour is the length of the day night cycle of the planet Mars. So it comes as good news that our circadian rhythm could adapt if we ever wanted to live on Mars.
The Sleep Wake Cycle – Patterns of Sleepiness & Alertness
During the 24 hour cycle of our circadian rhythm we have natural peaks and dips in alertness.
We tend to have a peak in alertness around 10am and during the evening before we go to bed. The evening peak of alertness is known as the forbidden zone, because going to bed at this time makes it hard to fall asleep.
We have a dip in alertness early afternoon known as the post lunch dip. Our lowest point of alertness occurs during the early hours of the morning, when we would be in the deepest stage of sleep.
Since drivers are safer when they are more alert, there are more road accidents during these dips, and less at the peaks of alertness.
Morning Larks, Night Owls and Hummingbirds
While everyone has a 24 hour circadian rhythm aligned to the sleep wake cycle, not everyone reacts to the time of day in the same way.
People can be categorized into 3 areas, morning larks, night owls and hummingbirds:
- Morning Larks – Tend to wake up earlier and go to bed earlier
- Night Owls – Tend to wake up later and also go to bed later
- Hummingbirds – Have neither tendency to wake up or fall asleep earlier or later
Two thirds of people are hummingbirds. Out of the final third, there are twice as many night owls as there are morning larks.
Teenagers are biologically inclined to become owls, and social activities late in the night can further enforce this. Older people are more naturally inclined to become larks. There is also a genetic disposition to whether you are a lark, owl or hummingbird.
To find out whether you’re classified as a night owl, morning lark or hummingbird, take the Morningness Eveningness Questionnaire.
Circadian Rhythm Sleep Disorders
If you sleep in or wake up a few hours early, there usually isn’t a problem. But it does become an issue when you find yourself unable to wake up or unable to stay awake for work or social commitments. This is when the timing for your sleep becomes an issue, and you may be diagnosed with a circadian rhythm sleep disorder.
Circadian rhythm sleep disorders come in two flavors:
- Advanced sleep phase syndrome – Falling asleep and waking up too early
- Delayed sleep phase syndrome – Falling asleep and waking up too late
The cure for both circadian rhythm sleep disorders is to shift the phasing of your sleep either forward or back. This requires you to arrange your zeitgebers either earlier in the day if you want to sleep earlier, or later in the day if you want to sleep later.
By shifting your exposure of light, mealtimes and activities, you will be calibrating (entraining) your body clock by shifting the processes associated with the circadian rhythm either earlier or later in the day.
Circadian Rhythm Miscues
Jet lag occurs due to the confusion of your circadian rhythm to the different cues of your new time zone, where your meal times and exposure to daylight will be different.
Shift work also causes problems, especially if you start your work day at night and finish when the sun rises.
We share 98.5% of our DNA with chimpanzees. We weren’t designed to stay up at night because we can’t see in the dark. And our circadian rhythm has never before needed to adapt to new time zones. Both electric lighting and our flying abilities are very new developments, considering humans have been walking the earth for 200,000 years.
Whether or not our circadian rhythm will evolve to adapt in the next thousand years is yet to be seen. But for now at least, we have the biology of the primitive animal we have evolved from.
The more we understand, and help the natural processes aligned with the circadian rhythm, the easier time we will have getting sleep.