How long is a day? Your body’s answer is about a day or circadian (which means the same thing). You may think that the answer should be 24 hours and to the clock it is. To your body and its fluctuating energy rhythms, however, the clock on the wall is secondary. Your body sets itself to sun time which, for some of us, doesn’t match up well with the clock on the wall.
The average human body-clock would run to about 24 hours and 13 minutes if it didn’t have the sun or other zeitgebers (time giver in German) to synchronise and reset with.
Sun time is way less regimented than clock time – and I’m not talking about seasonal differences in day length. I am fascinated by the fact that a day, solar, celestial, sidereal or otherwise, is only ever the exact same length a couple of times a year and actually runs fast or slow by as much as 16 minutes at certain times. This sun time variation is to do with irregularities in our orbit around our sun. Anyway, back to the rhythms.
I’m assuming you’ve heard of the (day-long-ish) circadian rhythm (or body-clock) and if you know one thing, you know that it governs your sleep time preferences but it does a lot besides that and there are some other, less well known, rhythms that overlap with the circadian.
Do you have an energy dip early or mid afternoon? You’re likely to if you’re a mid-chronotype. Larks and owls have their dips at other times but across all chronotypes (averagely) the energy dip is about 7 hours after you wake up. For most of us, with a natural healthy sleep regime, your energy rises after you wake, dips 7 hours later, then rises again into the late afternoon or evening.
In more detail, these rhythms are different for specific energy uses (cognitive, physical, creative, emotional, social etc). Your best time of day for solving complex problems is unlikely to be the same time of day as your best cardiovascular performance or for your creative insights. The Day Crafting Body-clock Workbook helps you work out what this means for you and put it into practice.
There are other daily cycles, physiological, genetic and hormonal, that also split the day into two, which is what circasemidian means. For example, baseline stress, blood pressure and body temperature all have a sub cycle within the daily circadian cycle.
This image is derived from the sleep cycles my apple watch detects when I wear it overnight. You most likely have five sleep cycles too. They don’t always come out this clearly but with some simple calculations, you can work out that each is about 90 minutes long. This is an example of an ultradian rhythm. This is much shorter than circasemidian; multiple ultradian rhythms fit into a day both during your sleeping and waking hours.
How can you sync with or utilise your ultradian cycle? This is not an easy question to answer. Some sleep apps claim to monitor your ultradian rhythms during the night and wake you up at the optimum, light sleep moment at the end (presumably) of your fifth cycle giving you the (average) 8 hours that you need and getting your day off to a good start. Most people don’t need this. If you need an alarm to wake you up then you’ve got bigger problems as this is one sign of circadian misalignment.
It could be an idea to shape your work/rest cycles to your ultradian rhythm (which could be between 90 and 120 minutes). There is no harm in trying if you can also measure the results but I don’t think our ultradian rhythms are that regular during a day or over multiple days. No two of my nights in a row are ever as clear as the image above; in other words, I think we do have an ultradian work/rest cycle during our working hours but it is subtle and irregular so I find it is better to aim for breaks at roughly 90 ish minute intervals but make adjustments to this on the fly.
I remember the details of a calorie intake experiment that followed ultradian rhythms where North American forest fire-fighters divided their food into smaller rations separated by 90 minutes during the day rather than three bigger meals. The result was that their physical energy across their working day was much more even. Our ultradian rhythm is easily affected by our food intake and the subsequent hormonal shifts of our (circasemidian) digestive system.
There are some other ultradian rhythms longer than 90 to 120 minutes but shorter than circasemidian such as the nasal cycle (which isn’t a bike you steer with your nose) and the growth hormone cycle (4 and 3 hours (ish) respectively).
If you have interventions that work with the subtleties of your ultradian rhythm, I’d love to hear more.
In contrast to ultradian, these are rhythms that are longer than a day. The most common infradian rhythm is the menstrual cycle (for some, this is a huge influence over many of the other rhythms listed above). Another fairly well understood rhythm is the link between the season and depression levels; usually when light levels are lower. Some babies and much older people fall into infradian sleep rhythms and struggle to synchronise to their circadian rhythm.
Do you experience any infradian rhythms in your life that aren’t listed here?