[MUSIC PLAYING] NARRATOR: Why do most people get sleepy in the afternoon? Why do they feel alert and energized around 10:00 in the morning? Well, there is something that guides how we feel throughout the day called the circadian rhythm. This is a rhythm that is naturally occurring and is built into the systems of the human body along with plants and animals. The term circadian rhythm was developed in the 1950s by a man named Franz Halberg who derived the term from the Latin words circa, which means about, and diem, which means day. Therefore, the word literally translates into about a day rhythm. 

Even though the term is relatively modern, scientists have been aware of this rhythm for thousands of years, both the Egyptians and the Greeks had some understanding of the circadian rhythm. There are several criteria that a biological circadian rhythm must follow. 

First, it must last around 24 hours. And, second, the rhythm can be reset by exposure to light and heat. This phenomenon can be seen as we travel across time zones and get jet lag as our bodies slowly adapt to the new local time. 

Recently, it has been concluded that disruptions to our circadian rhythm can cause health problems. Chronic disruptions of the circadian rhythm has been shown to cause fatigue, insomnia, and is at times linked to obesity and diabetes. At this point, you might be wondering what happens during this 24-hour cycle? What exactly happens during this rhythm? 

If you take a normal day and break it down from sunrise to sunrise, it may look something like this. In the morning time, our body experiences its sharpest rise in blood pressure and a reduction in the secretion of melatonin. These occurrences cause our bodies to feel awake. It has been recently discovered that enjoying a cup of coffee first thing might be counterproductive to your day. 

In fact, early in the morning, our bodies have the highest level of a hormone called cortisol, which makes us naturally feel more awake. The caffeine and coffee actually interferes with cortisol production. Your body makes less of it accordingly, and the body then relies more on the caffeine to feel awake. By this logic, then, it is better to have a cup of joe in the early afternoon so that the body does not become as dependent on caffeine to feel alert. 

In the afternoon, our bodies have the best coordination, reaction time, cardiovascular efficiency, and muscular strength. This gives us an explanation as to why so many world records are broken in the late afternoon. In the evening time, our body temperature begins to increase along with the secretion of melatonin, signaling to our bodies that it's nighttime and it's time to go to sleep. 

We hit our deepest stage of sleep around 2:00 in the morning. Using this information is key to living a productive and happy life. After all, it's part of our nature. And the fight against nature is not easily won. Anyways, it's about 5:00 in the afternoon right now, so I'm going to go ahead and get ready for my afternoon run. Thank you for listening, and please click the button to subscribe.