Why is more oxygen needed at nighttime?

Rods in the eyes are responsible for vision in low-light conditions and require more oxygen to function effectively, especially when cones, which are responsible for color vision and are more effective in bright light, become inactive in the dark. As the environment darkens, rods become the primary photoreceptor. Their increased activity necessitates a higher oxygen demand to support the metabolic processes involved in phototransduction—the conversion of light into electrical signals for the brain.

In contrast to the misconception that increased oxygen demand is tied to higher activity from cones, the dark conditions downregulate cone activity while rods take over. As a result, the need for oxygen intensifies for rods to maintain optimal functionality during nighttime vision. Understanding the relationship between light conditions and the function of ocular photoreceptors is crucial for grasping why oxygen requirements change throughout the day and night cycle.