First and foremost, the circadian rhythm is our biological clock telling us, among other things, when to wake up and when to go to sleep. And the clock is calibrated by natural light, such as sunlight, over a 24 hour day.
Indeed, the hypothalamus, located in your brain, controls your circadian clock. Furthermore, the hypothalamus has a bunch of cells called the suprachiasmatic nucleus, or SCN, which does the actual controlling. Moreover, the SCN gets help from optical nerves located in the eye’s retina. As a result, the sunlight that you see or don’t see, helps the SCN calibrate itself with the 24 hour clock.
So when you see the natural light in the morning, the SCN causes your body temperature to rise, and your blood pressure along with your heart rate to increase. Most of all, the SCN stops making the hormone melatonin. As a result, you start to feel sleepy.
Our body clock is, in most cases, set so that we sleep around 11 PM at night and wake up around 7 AM in the morning. Incidentally, people are most sleepy between 2 to 4 AM in the morning and between 2 to 3 PM in the afternoon.
But not everyone follows this pattern. In fact, the body clock, of some people, starts producing melatonin two or more hours earlier in the evening. As a result, these people get tired and sleepy before the others. And they also wake up much earlier in the morning than others. Also, they tend to feel very sleepy in the afternoon. Indeed, the elderly are more prone to this behavior.
On the other hand, the body clock in others starts producing melatonin 2 or more hours later causing them to feel tired and sleepy much later into the night. And they wake up much later in the morning. Unfortunately, constraints such as getting to work by 9 AM, causes these people not to get the full 7 to 8 hours of sleep. As a result, their sleep loss is not good for their health (see infographic).