WHY IS IT IMPORTANT TO UNDERSTAND BIOLOGICAL CLOCK?

Why do we feel sleepy in the evening even when we are not particularly tired? Why are we at our slowest around three in the morning? Why do most people face their highest risk of a heart attack or stroke during the morning hours? Although these may seem like separate phenomena, they are all connected by the same mechanism — our biological clock.

In the previous article, we explained what the biological clock is and how our bodies recognize the transition between day and night through the internal circadian rhythm. However, a logical question arises: why does this matter in the first place?

Throughout each day, our bodies undergo a series of carefully coordinated changes that affect alertness, concentration, body temperature, blood pressure, hormone secretion, and many other functions. In other words, not every hour of the day is equally suitable for every activity.

Of course, this rhythm is not entirely fixed, but the fundamental principle remains the same. That is why it is important to understand the processes underlying our daily variations.

What Causes Evening Sleepiness?

Fatigue and sleepiness are concepts we often use interchangeably, but there is a difference. Fatigue is a feeling of physical or mental exhaustion, whereas sleepiness is a biological signal that the body needs sleep. This is why we can feel extremely tired yet remain unable to fall asleep for hours. Conversely, even after a relatively restful day, we may experience pronounced sleepiness in the evening because our biological clock has reached the point at which it begins preparing the body for rest.

One of the most important players in this process is melatonin. As night approaches and exposure to light decreases, the biological clock stimulates its release. Melatonin does not induce sleep in the same way as a sleeping pill. Instead, it signals to the body that biological night has begun and increases our natural propensity to sleep. Melatonin levels rise throughout the night and decline again toward morning as the body prepares to wake up.

However, this natural signal can easily be disrupted. Blue light emitted by phones, computers, and televisions, along with scrolling and mental stimulation late in the evening, can delay melatonin release and shift the onset of sleepiness to a later time. That is why we often feel exhausted after a demanding day but stop feeling sleepy once we start using our phones. The body receives signals that it is still daytime, delaying the onset of biological night and disrupting sleep quality, particularly its deeper stages, which are essential for recovery.

The Body’s Internal Wake-Up Alarm

The body begins preparing for wakefulness long before we open our eyes.

Cortisol plays a major role in this process. Although we most often associate it with stress, cortisol is also an integral part of our natural daily rhythm. Its levels are lowest during the night, allowing the body to rest and recover. Toward morning, cortisol levels gradually begin to rise, followed shortly after waking by a characteristic increase known as the cortisol awakening response (CAR).

This increase is not a sign that we are stressed. Rather, it is a normal physiological response that helps the body transition from sleep to wakefulness. Cortisol mobilizes energy, increases alertness, and prepares the body to meet the physical and mental demands of a new day.

When we wake up abruptly, particularly after insufficient sleep or at a time when the body has not yet initiated its natural waking process, the transition from sleep to wakefulness can be difficult. This may result in a feeling of heaviness, sluggishness, disorientation, and the familiar sensation that we need some time to fully come around.

In other words, an alarm clock can wake our conscious mind, but the body's biological systems sometimes need more time to complete their natural transition from nighttime to daytime mode.

Problems arise when this natural rhythm is repeatedly disrupted by insufficient sleep, staying up late while exposed to blue light, and prolonged stress. This can disturb the delicate balance between rest and activity, contributing to poorer sleep quality, persistent exhaustion, and a reduced ability to recover.

To support our natural rhythm, the most important things are to allow enough time for sleep and maintain as consistent a sleep schedule as possible. Going to bed at approximately the same time each night and reducing exposure to external stimuli before bedtime help the internal clock maintain a regular rhythm. Staying up late, on the other hand, can shift the entire cycle of preparation for sleep and waking.

The Body Is Not Equally “Warm” Throughout the Day

Another indicator of the biological clock at work is body temperature. Although it is maintained within a very narrow range, it nevertheless follows predictable daily fluctuations.

During the night, body temperature gradually decreases, providing one of the signals that facilitate falling asleep. It reaches its lowest levels toward morning, precisely when the body is naturally geared toward rest. Afterward, it begins to rise gradually, typically reaching its peak in the late afternoon, when the body is better prepared for physical activity.

This rhythm can also explain some everyday experiences that we do not usually associate with the biological clock. For example, someone who is awake at three or four in the morning may feel unusually cold even when the room is not particularly cold. During this period, core body temperature is naturally at its lowest, and the body is operating in a “night mode” characterized by reduced activity.

The same principle helps explain why elevated body temperature during many infections often becomes more pronounced in the second half of the day. This is not necessarily a sign that the illness itself is worsening; it also reflects the natural daily rhythm of body temperature, which influences how the body responds and how we experience certain symptoms.

When Does Our Brain Function Best?

Cognitive functions are the abilities that allow us to receive, process, and use information. They include attention, concentration, learning, memory, planning, decision-making, reaction speed, and the many other mental abilities we use every day.

It is important to note that there is no single time of day when all cognitive abilities simultaneously reach their peak. Different functions perform best at different times. For most people, alertness and the ability to sustain attention increase throughout the morning. More complex executive functions, such as planning and decision-making, often reach optimal levels around midday, while reaction speed in many people peaks later in the day.

In contrast, late at night and in the very early morning, overall cognitive performance is generally reduced. During these periods, attention, reaction speed, and the ability to make complex decisions tend to decline, which is particularly important in situations requiring a high degree of precision and safety. This is precisely why night shifts, prolonged duty periods, and driving late at night carry a greater risk of errors and accidents.

Nevertheless, individual differences exist. These differences are partly determined by our chronotypes — our natural tendencies to be more active and productive earlier or later in the day. Some people are morning types, while others function more easily later in the day. Chronotype influences when we feel most alert and when we perform certain tasks most efficiently, but the fundamental principles of the circadian rhythm remain broadly similar for most people.

Blood Pressure Also Follows a Daily Rhythm

The biological clock also influences the functioning of the heart and blood vessels.

In healthy individuals, blood pressure naturally decreases by approximately 10–20% during the night. This is a normal pattern known as nocturnal dipping. After waking, blood pressure gradually rises, followed by a more pronounced increase that prepares the body to transition from rest to everyday activity.

Throughout the day, blood pressure remains relatively stable, with minor fluctuations, before gradually decreasing again in the evening.

This rhythm is more than just an interesting physiological fact. People who do not experience the expected nighttime drop in blood pressure, or whose blood pressure actually rises during the night, have an increased risk of cardiovascular disease. Similarly, an excessively pronounced morning surge in blood pressure is associated with a higher risk of heart attack and stroke.

Conclusion

Melatonin, cortisol, body temperature, cognitive functions, and blood pressure are just individual components of a larger process that regulates our biological rhythms. The purpose of this coordinated mechanism is to ensure that the body is ready for rest, mental work, or physical activity at the appropriate time.

Understanding these natural variations does not mean having to live according to a rigid schedule. Shift work, travel, family responsibilities, and occasional sleepless nights are unavoidable realities for many people. However, when the mismatch between the biological clock and our lifestyle becomes the rule rather than the exception, the body will eventually begin to feel the effects.

That is precisely why understanding our own biological rhythms can be a valuable tool for better daily planning, improved sleep quality, and more effective performance in everyday activities.

If different systems in our bodies have periods when they function most efficiently, a logical question arises: Can we organize our day in a way that allows us to make the most of them?

That is exactly what the next article will explore.

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