Circadian Rhythms

The time has come for circadian rhythms.

Yesterday, three scientists—Jeffrey Hall, Michael Rosbash, and Michael Young—were awarded the Nobel Prize in medicine for their work on circadian biology, which is the field of science dedicated to the internal clock on which our bodies run. The ticking of this 24-hour cycle determines when we get tired and hungry, as well as controlling everything from the (very real) phenomena of jet lag to female ovulation, when we’re more likely to have a heart attack, and the time of day we learn best.

“Their discoveries explain how plants, animals and humans adapt their biological rhythm so that it is synchronized with the Earth’s revolutions,” said the Nobel jury.

What are circadian rhythms?

Circadian rhythms are evolution’s answer to the daily cycle between day and night. Life can be pretty unstable, but if biology on Earth can rely on one thing, it’s that the sun will dependably rise and set every day. If you can anticipate change, you can get ahead of it, so early life forms adapted to predict when it would get lighter and hotter, and when it would get darker and cooler over a 24-hour cycle. That meant less sunburns (which are lethal when you’re only one-cell big), more efficient photosynthesis, and eventually the ability to anticipate the activity of your predators or prey.

Fast forward a few millennia, and it also meant our human bodies evolved with genetic clocks inside each cell. One of the biggest challenges for large organisms like us is making sure all our internal clocks run in sync with one another. This coordination allows female pituitaries and ovaries to match up when triggering ovulation; our pancreas, gut, and hypothalamus to link up to make us both hungry and ready to digest; and sleep to be timed to when our muscles are ready to cool down and enable healing, as well as when our brains are most free for maintenance and memory formation.

Well, that’s how nature had worked it out, anyway. In the 21st century, we’ve messed up that internal coordination with things like artificial light, fast-food binges at 2am, shift work, and early college-class schedules. These forced schedules mess with our body’s natural ebb and flow of when we should be sleeping and eating according to where the sun is in the sky, and it’s wrecking havoc on our systems.

Circadian disruptions are likely contributors to the obesity epidemic, the US’s cancer rates, the autism epidemic, the increase in Alzheimer’s and other dementias, and the growing dissatisfaction within public education. Because every part of our body has been imbued with an innate circadian rhythm to keep all the clocks ticking in sync, every piece of us is susceptible to circadian disruption.

hings that are controlled by our circadian rhythms

To give you an idea of how pervasive circadian biology is to our lives, here’s a quick list of the ways it affects us:

Jetlag: When the time of day suddenly artificially changes, some of your organs catch up faster than others. During this adjustment period, your insides are aligned to different clocks running at different speeds. As a result, they can’t do their jobs as well, and you feel funky (or fall asleep in the back of cabs).

Fertility: Your sex drive has a circadian rhythm, and so do your sex hormones. For example, ovulation is under circadian control to happen in the early morning. I’m told that this was first documented when IVF was invented in France and they found women flying in from the States had lower success rates: Turns out jetlag was messing with their ovulation! (By the way, so does shift work.)

Mental focus: You learn best at a time of day—but that optimum few hours is different for everyone. New brain cells, new synapses, sleep, and attention are all regulated by the time of day, so keeping a routine keeps them razor-sharp. (Fun fact: When you learn something, it gets a circadian time-stamp, and you recall it best at that time. So it’s always best to practice at the same time of day as you’ll perform.)

Nutrition: Circadian rhythms don’t just control when you get sleepy: They control when you get hungry, too. Just as your body absorbs information differently at different times of the day, it does the same with food. For example, at night, food converts into fat more readily than it does during the day.

Heart attacks: Your heart is used to dealing with stress during the day—there’s a lot to be anxious about nowadays, after all—but it recovers at night. As your body ramps up during the morning, your heart, still chilling out, gets strained, so most heart attacks occur in the early morning.

Aging: Finally, as you get older, your body has more trouble holding all of these rhythmic pieces together. The more they fall apart, the more your risk of age-related maladies increases. Getting bright sunlight in the day and having a dark, quiet space to sleep can help keep your circadian rhythms robust—and keep you alive longer.

The importance of circadian biology

And we wouldn’t know a lot of this if it weren’t for Hall, Rosbash, and Young. These newly minted Nobel laureates first showed that internal clockwork could be mapped on a schedule, which seeded an exponential expansion in time-focused biological discovery. The reason science on circadian rhythms has grown so substantially is that each time a biomedical field of science integrates circadian biology, every part of that field has to be re-examined to take the time of day into account. Compare this to fields not yet kissed by the circadian method, where samples are taken whenever it’s convenient for the researchers and lumped together in one, time-vague data point. This blurs variability across the day. Circadian biology is therefore an ever-growing library, in which these men wrote the first several mechanistic volumes.

This re-examination of other scientific fields through the lens of circadian rhythms has heralded the construction of “chronotherapy” in the medicine industry. If a drug is most toxic at a certain time of day, but most effective at another time of day, then timing the dose can mean you use smaller amount of the drug, get more bang for your buck, and have fewer side effects. This has made a big impact in cancer treatment and sleep therapy, for example, but it’s full potential is still in its early days.

We now also know that our circadian profiles are personal. Just as there is not one food that can grow a healthy society (I’m looking at you, refined flour), when it comes to going to bed or eating breakfast, there is not one time that works for everyone. Trying to hold on to old models forces a lot of people to suffer circadian disruption, which is both bad for their wellbeing in the moment and can potentially implicate their future medical costs. I believe we might come to optimize social obligations like work and school around individuals, because the alternative will be shown to be abusive to our individual biological makeup.

Another personalized area we’ll see changes in will be predictive medicine. When you track changes in a biomarker over time—such as blood pressure or cortisol levels—you can detect the deviations from your normal baselines not just for that day, but for that specific time of day. When something in your body changes—like getting sick or becoming pregnant—the shape of your biological day changes, too. Some chronobiologists are working to build predictive algorithms based on detecting personal deviations from the normal shape of the day, and using those deviations to predict future changes.

In research, we've found patterns for predicting cancer and surgical recovery, detecting pregnancy and predicting pregnancy outcomes, and for monitoring fertility, sleep, stress, and learning. Others in my field are predicting sepsis in hospitals and identifying people at risk for Alzheimer’s Disease—and helping them stave it off by strengthening their circadian rhythms. This predictive medicine, based on time-series analysis, will be cheaper, more accurate, and more personal that medicine could otherwise be.

There is still a mountain of work ahead of us before the world is optimized around its biological timing. But because of these Nobel laureates, and the attention their work will now be given, we finally might all get on the same rhythm.