Most of us go through life merrily unaware of nature’s clock that is controlling our primary life functions… until we go on vacation and jet lag strikes. There is nothing quite like that feeling of becoming completely, totally, overwhelmed by sleep in the middle of the day. It’s as if you’ve been put under a general anaesthetic, and a reminder that there are systems governing our body and its functions that are completely out of our conscious control. Our sleep cycle is just one function among many that runs on a 24 hour clock, and these systems are all described by the term circadian rhythm. I was surprised to find that it was coined quite recently, in the 1950s, as the Latin roots circa (around) and diem (day) suggested to me a more ancient origin. I’ll just take you through a quick tour of the topic, and go off on a few interesting tangents along the way.
To start with, let’s take a look at an excellent graphic found on Wikipedia that shows the timing of a typical person’s circadian rhythms over one 24 hour day (Figure 1). I had a chuckle when I found this, as it was just after 8:30am, upon returning to my desk.
Circadian systems appear to be ancient, extending back to the dawn of time, when the first primitive single cell organisms developed mechanisms to protect their DNA from the high levels of UV light found during the day. These early, primitive and simple clock mechanisms have stayed with life throughout its billion year evolution, as even today each living cell retains some type of clock mechanism that can run autonomously. More complex biological clock mechanisms are built on these fundamental cell clocks, with cells able to send time synchronisation signals to neighbouring cells electrically. Some types of cells are able to communicate with the endocrine system. It in turn can send out hormone signals to widely broadcast more clock information throughout the body, to specific subsystems such as the digestive system.
The immediate question that comes to mind is, how does our body know what time it is? The most obvious answer is the sun, but there are other external cues that are used to keep our systems in synch; these are known as zeitgebers, literally time givers in German. As discussed in my Science Break article “Plant hydraulics” (Kuhn, 2013), even plants are capable of fairly rapid response to external stimuli, and of course animals are too. But those responses are not circadian; in order to meet the criteria, the system must more or less lock in to a 24 hour clock, and only adjust gradually to external stimuli. Imagine if we were like birds, and we could just put sacks over our heads to fall to sleep, any place, any time. Actually, I don’t even know if that’s true, but the owners of some kinds of pet birds just put a sack over the cage and they go silent, and act as if it’s night time. But maybe they’re not sleeping, they’re probably just programmed to stay still and quiet when it’s dark, given that they are very vulnerable to predators when they can’t see well enough to fly away. Anyway, circadian rhythms must be relatively unaffected by shorter term external changes, but able to gradually adjust over time, described by the term entrainment. This is why it’s so tough to get over jet lag. I’ve heard that our sleep systems can change about one hour per day, meaning if you return from Europe to Calgary, it will take 8 days for your sleep patterns to fully adjust to the 8 hour time difference.
But back to light. The primary zeitgeber is the light received by our eyes. Animals that live in certain extremely specialised environments – caves, deep ocean abysses, the Arctic, etc. – have evolved to rely on alternate zeitgebers in the absence of light and even eyes in some cases. But most animals including humans rely on light as the primary zeitgeber. The system is designed to work best with light conditions that, as you’d expect, are similar to outdoor daytime – overhead, with a high intensity of up around 1000 lux, and more towards the blue end of the spectrum. When dedicated photoreceptive ganglion cells in the retina receive light like this, then they send a photopigment called melanopsin along the retinohypothalamic tract directly to some 20,000 suprachiasmatic nucleus (SCN) cells in two clusters within the hypothalamus. These signals are not a one-time deal, they are continual throughout the day and vary according to light levels, and the SCN uses this to determine the length of the day (and conversely the night). Based on these estimates, the SCN passes instructions on to the pineal gland which is located in the epithalamus. The pineal gland produces and distributes the appropriate amounts of the hormone melatonin, basically sending it out at night and not during the day.
The differing levels of melatonin circulating through the body then synchronise and control the clock mechanisms in various systems – these include really all the basic functions of the body, such as defecation, eating/ hunger, thirst, sleep, mental alertness, physical preparedness, etc. I would also include more specialised and selective processes such as the synchronisation of breast milk production in a mother and hunger and digestive acids in the baby, although there are additional, non-circadian systems at play here as well. There is an entire suite of ultradian (periods less than 24 hours) and infradian (periods greater than 24 hours) systems operating in our bodies as well – the menstrual cycle is a great example of the latter, and actually most of the systems mentioned at the beginning of this paragraph really are a combination of circadian and ultradian rhythms. For example, the circadian clock tells the body it’s daytime, and then an ultradian clock works off that and tells us when we’re hungry throughout the day, triggers the release of stomach acids appropriately, and so on. I would also venture to say sexual activity, or at least appetite for it, is affected by circadian rhythms – I find it very suspicious that teenagers are so darn alert exactly when the guardians of their virtue (i.e. the parents) can’t stay awake. But what did virtue ever do in terms of helping the propagation of the species? Nothing!
A big issue in the modern world is the prevalence of disrupted circadian rhythms. Shift work and constant travel wreak havoc with a body’s circadian rhythms, but other trappings of our lifestyles are problematic as well. People in less developed societies without artificial light and the ability to jet around the world conduct their lives in environments that the circadian systems are designed to operate in – no flashing strobe lights in bars, no sleeping in until noon in curtained rooms, just the regular rising and setting of the sun. Some of the best sleeps I’ve ever had have been while camping – you just feel sleepy when it gets dark, and fall asleep… in the morning you’re woken by the sun bursting through the tent walls. Of course nowadays if I tried to sleep in a tent it would be a futile exercise – my aging, pampered body needs a good mattress, but that’s beside the point! I remember a week I spent in Bali in a thatched roof cabin perched high above terraced rice paddies, with only candles for light. As soon as the sun set, which is early near the equator, it was all I could do to stay awake. I am sure people who live in these societies enjoy regular and satisfying sleeps almost every night. We on the other hand, are bombarded by artificial light when it should be dark, and are often inside at work in the morning right when we should be getting a big dose of light to turn off the production of melatonin.
Disruptions of circadian rhythms, especially sleep patterns, have been linked to all sorts of physical and emotional problems, ranging from the fatigue and upset stomach of jet lag, to obesity, all the way to serious conditions such as bipolar disorder and cardiovascular disease. A few articles ago I was touting the benefits of LED lighting, yet it’s really interesting to note that exposure to LED light will on average suppress melatonin production five times more than with other forms of light – something to think about. On the one hand I think it’s important to consider all this if you are experiencing difficulty sleeping, but on the other hand I’m worried that I may be offering hypochondriacs out there an entirely new playground to feel miserable in. For me what is most fascinating is that circadian rhythms are another excellent example of just how complex our bodily systems really are.
Key Search Words
Circadian, melatonin, entrainment, ultradian, infradian, melanopsin, zeitgeber
Kuhn, O. (2013, February). Plant hydraulics. RECORDER, 38(2).
National Institute of General Medical Sciences. (2013, June 12). Circadian Rhythms Fact Sheet. Retrieved November 26, 2013, from National Institute of General Medical Sciences: http://www.nigms.nih.gov/Education/Pages/Factsheet_CircadianRhythms.aspx
National Sleep Foundation. (2013). Sleep Drive and Your Body Clock. Retrieved November 26, 2013, from National Sleep Foundation: http://www.sleepfoundation.org/article/sleep-topics/sleep-drive-and-your-body-clock
Wikimedia Foundation, Inc. (2013, November 17). Circadian rhythm. Retrieved November 25, 2013, from Wikipedia: http://en.wikipedia.org/wiki/Circadian_rhythm