Sleep deprivation reduces alertness and slows the brain’s processing, so performance on tasks that require sustained attention, quick reactions, and accurate decision making drops. When you’re sleep-deprived, the neural networks that support attention and executive control don’t function at full capacity, leading to slower responses, more mistakes, and difficulties solving problems. Memory is also affected because sleep supports memory consolidation, so lacking sleep weakens that process and further lowers how well you perform on tasks that rely on memory. By contrast, light entering the eye is a physical process governed by the pupil and ambient light, not by how tired you are, and the difference threshold is a basic sensory measure that isn’t the primary way sleep loss shows up. So the most general and directly observable impact of sleep deprivation is on performance—the overall ability to perform tasks effectively.