Why do we have circadian rhythms?

Why have 24-hour rhythms?

Multitasking can either increase or decrease your productivity, depending on the tasks you are trying to combine. Driving while talking on the phone, watching TV while working on a project, running a marathon while checking your Instagram – we may have done this, but we don’t do it on a regular basis because it is not the safest nor the most productive way to accomplish a task. But there are some acts that truly go together – listening to music while working on an assignment, eating dinner while talking to friends or family across the table. In fact, some acts go hand in hand so well that we feel happy and productive doing them together.  Overall, we tend to combine compatible acts, and attempt to keep incompatible acts apart.

Similarly, our bodies do a lot of different functions on a daily basis. These include eating or drinking, actively looking for food or companionship, and sleeping. Underlying these rhythms in nutrition, physical activity, and sleep are numerous physiological functions. For example, the sleep hormone melatonin helps us fall asleep, and when we are asleep our muscles are less responsive to brain signals, so we don’t act out our dreams by walking or running around – that would be dangerous. When we are awake, levels of the stress hormone cortisol are slightly higher than when we are asleep. High cortisol levels make us more alert and active. When we play a sport, we need our muscles to respond quickly and robustly to signals from our brain so that we can coordinate our movements. We don’t want sleep hormones circulating during this time. Overall, our body tries to coordinate physiology (e.g., levels of hormones, metabolism, and brain function) so that body functions necessary for a particular behavior, such as playing sports, occur together. In contrast, our bodies try to segregate body functions necessary for contradictory behaviors, such as playing sports and sleep.  Our circadian rhythms play important roles in these processes. Circadian clocks in different organs and brain regions set in motion programs to ensure that hormones, nutrition, brain function, muscle function, etc. that are necessary for sleep occur at night, whereas those supporting physical activity and alertness occur during the day.

If we didn’t have circadian rhythms, our daily life would be very unpredictable. If you have ever taken care of a new born baby you would know how is life without a circadian clock. Newborn babies are still developing a fully functional circadian clock. So, they don’t have a strong rhythm to the sleep/wake cycle. As a result, they feel hungry every 2–4 hours and cry in hunger. After feeding they go back to sleep until they feel hungry again in a few hours. Imagine if we all were like babies. It would be hard to get together with friends or work in an office, as at any given time about half would be sleepy and the other half would be awake or hungry.

Another reason we have circadian rhythms is to conserve energy. Imagine a house in which the kitchen stove is always burning, the dishwasher is always running, the washer and dryers never stop, the vacuum cleaner and lawnmowers work around the clock. What a waste of energy. It makes more sense to turn these devices on only when they are needed. That is exactly what happens in our body. It is not worth producing digestive juices throughout the 24-hour day when we eat only a few times a day at predictable times. There is also no benefit to having our muscles at top performance levels when we sleep. Besides, when an organ or brain region does work, waste products are generated. These waste products can damage our tissues, and therefore must be neutralized and removed from our body. So, having circadian rhythms reduces the overall energy usage and helps to reduce the accumulation of tissue waste and tissue damage.

One more reason for having a circadian clock is to efficiently repair and rejuvenate our body. Circadian clocks in different parts of the body and brain work together to repair damaged cells and to make new cells. At night when we go to sleep, our brain is detoxified, our gut and skin generate new cells to replace damaged ones, new blood cells exit the bone marrow and enter the circulation, and toxic products from muscle and liver are collected in urine. Numerous such repair and rejuvenation processes occur in different tissues at specific times. Just as a highway cannot be efficiently repaired if traffic is allowed to flow, our circadian rhythms take advantage of downtime to activate daily rejuvenation processes.

Circadian rhythms are essentially timing programs ingrained in our body and mind to ensure that basic functions are efficiently performed on a daily basis. They help us sleep better, work better, and cleanse our body better by timing the production of certain hormones, and the activation of certain cell functions in different parts of our body and brain.

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