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The autonomic nervous system (ANS) is a crucial controller of the body's involuntary functions, including the regulation of pupil size. It is divided into two main branches: the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system prepares the body for 'fight or flight' responses, and one of its actions is to dilate the pupils in situations of stress or heightened alertness. On the other hand, the parasympathetic nervous system orchestrates 'rest and digest' activities. It is responsible for making the pupils constrict, promoting a calmer state that is conducive to rest.

This intricate balance is constantly adjusted by the ANS in response to various internal and external stimuli. For example, stress, excitement, or fear can trigger the sympathetic branch, leading to larger pupils. Conversely, relaxation, satisfaction, or a state of calm can activate the parasympathetic system, resulting in smaller pupils. Beyond light exposure, these physiological changes are key factors affecting pupil size.

The process of pupil dilation and constriction is known as pupillary response, and it occurs primarily to regulate the amount of light that enters the eye. Dilation (mydriasis) is the widening of the pupils, which allows more light to enter during low-light conditions or in response to certain emotions, substances, or medical conditions. Constriction (miosis), the narrowing of the pupils, helps reduce light entry in bright environments or as part of a parasympathetic 'rest and digest' response.

Several factors can alter this dynamic, such as cognitive load, meaning that when we concentrate or engage in complex thought, our pupils might dilate. Moreover, certain medications, recreational drugs, and poisons can affect the muscles that control the pupil size - either causing them to involuntarily contract or relax.

The eye's response to light is a fundamental visual reflex known as the pupillary light reflex. Specialized cells in the retina detect light intensity and send signals via the optic nerve to the brain, which then commands the pupil to adjust its size accordingly. In bright conditions, the pupils constrict to limit the amount of light hitting the sensitive retina. In darker settings, the pupils dilate to allow more light to enter, improving vision.

However, light is not the sole influencer. Cognitive and emotional responses, like those associated with solving complex problems or feeling surprise, can also modify pupil size. This is due to the involvement of higher brain functions that impact the ANS, thus demonstrating the sophisticated interplay between various physiological systems and the environment in regulating eye response to light.