Why do we feel sleepy at certain times and sometimes wake up before the alarm rings? Why do our energy and concentration levels vary throughout the day? These phenomena are linked to an internal system known as the biological clock, which helps the body regulate its functions and adapt to the cycle of day and night. What Is the Biological Clock? The biological clock is an internal timing system that regulates circadian rhythms—physical, mental, and behavioral changes that follow a cycle of approximately twenty-four hours. These include changes in sleep and wakefulness, the secretion of certain hormones, and body temperature. This system is not confined to a single organ. Many tissues contain cellular clocks coordinated by a central clock in the brain. Located in the hypothalamus, this central clock is known as the suprachiasmatic nucleus. Light plays a fundamental role in adjusting this internal timing system and synchronizing it with the external environment. Light and Melatonin: Signals That Regulate Sleep Information about light travels from the eyes to the brain’s timing centers. As darkness falls, the secretion of melatonin increases, signaling the body to prepare for sleep. Exposure to bright light in the evening, including artificial lighting and screens, may interfere with this signal and delay the body’s preparation for sleep. The effect varies according to the timing, intensity, and duration of light exposure. Therefore, it is not limited to smartphone use; the surrounding light environment also matters. Why Does Sleepiness Increase the Longer We Stay Awake? Our need for sleep does not depend on the biological clock alone. Sleep pressure gradually builds up while we are awake and decreases during sleep. One substance involved in this process is adenosine, which accumulates during wakefulness and contributes to feelings of sleepiness. Sleep timing is therefore determined by the interaction of two factors: an internal rhythm that influences periods of wakefulness and sleep, and an accumulating need for rest that increases with the number of hours spent awake. How Is Internal Timing Disrupted? Circadian rhythms can be affected by travel across time zones, shift work, or frequent changes in sleep schedules. In these situations, the body’s internal timing may not align with scheduled periods of work, activity, and rest, making it more difficult to sleep at the desired time. Jet lag illustrates this phenomenon. A person arrives in a new country while their internal clock temporarily remains aligned with the previous location’s time zone, before gradually adapting to new environmental cues. The Importance of Understanding the Biological Clock Understanding this system helps us recognize that sleep is a regulated biological process whose timing is influenced by light, daily habits, and the duration of wakefulness. It also explains why energy levels vary throughout the day and why adapting immediately to major changes in sleep schedules can be difficult. Respecting the body’s daily rhythm begins with understanding the relationship between light, wakefulness, and rest, and recognizing sleep as an essential function that supports a well-regulated daily life. Almustaqbal University – The First University in Iraq