Cortisol helps you wake up. Melatonin helps you fall asleep. But why do these hormones peak at different times for different people?

Part of the answer lies in your genes. Another part lies in light. Together, they shape your circadian rhythm, the internal timing system that regulates sleep, energy, hormones, and many other processes across the day. When light enters your eyes, special nerve cells in the retina detect overall brightness rather than images. These cells send signals to a small region in the hypothalamus called the suprachiasmatic nucleus, often described as the brain’s master clock. This clock coordinates timing across the body.

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Early Birds, Night Owls, and Everyone in Between

Some people wake up early and feel alert almost immediately. Others struggle in the morning but feel focused and energetic late into the evening. These patterns are commonly called being an early bird or a night owl.

Research shows that these tendencies are partly genetic. Variations in clock-related genes influence how fast or slow a person’s internal clock runs. A slightly shorter internal cycle tends to push sleep earlier, while a slightly longer one pushes it later. This is why being a lark or an owl is not just a habit or a preference.

Most people, however, fall somewhere between the extremes. While genes help set the structure of your clock, they do not operate in isolation. This is where light becomes critical.

How Light Shapes Hormones

The circadian system cannot force sleep on command. Instead, it learns from patterns of light exposure. By tracking how bright your environment is, how long daylight lasts, and when light fades, the brain adjusts hormone timing each day. Morning light is the strongest signal. Exposure to bright light early in the day suppresses melatonin and increases cortisol release. Body temperature rises, heart rate increases, and alertness improves.

As daylight decreases later in the day, cortisol levels fall and melatonin production begins, often hours before sleep itself. This gradual hormonal shift prepares the body for rest.

For most of human history, this system operated under stable conditions. Days were bright and nights were dark. The circadian clock evolved to follow the sun, but it is flexible. That flexibility allows the clock to adjust to changing environments, though only within limits.

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How Electricity and Screens Changed the System

Electricity extended light exposure beyond sunset. Early artificial light sources were relatively dim and warm, so their effect on circadian timing was limited. Modern lighting and digital screens emit high levels of blue light, which strongly affects the circadian system. Blue light activates the same retinal nerve cells that respond to morning sunlight. When exposure occurs at night, the brain interprets this as a daytime signal. Melatonin release is delayed, cortisol remains elevated longer, and sleep timing shifts later.

Unlike sunlight, artificial light does not fade gradually. Screens remain bright and are often viewed at close range, increasing their biological impact.

Can You Change Your Sleep Type?

To a degree, but not completely. Genetics influence whether your clock naturally runs earlier or later. Light exposure, routines, and habits can shift that clock forward or backward, but they rarely erase underlying tendencies.

Early birds often experience a cortisol rise just before waking. Night owls tend to reach peak cortisol later in the morning. These differences shape how easy mornings or late nights feel.

Why Light Hygiene Matters

The circadian system depends on contrast between bright days and dim nights. Sunlight is far brighter than indoor lighting, even on cloudy days. Spending time outdoors during the day helps anchor the clock. At night, bright artificial light reduces contrast and delays melatonin release. Consistency in both sleep timing and light exposure helps the system function more smoothly.

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Your brain does not follow the clock on your wall. It follows light, guided by genes.