Hormonal

Cortical brain responses to sustained attention tasks decrease significantly with accrued sleep debt, whereas subcortical responses are primarily modulated by circadian rhythmicity and remain largely unaffected by sleep pressure.

If you are sleep-deprived, your brain's ability to sustain attention degrades specifically in cortical areas responsible for higher-order processing. This is not just 'feeling tired'; it is a measurable physiological change. To maintain performance, you must respect the circadian rhythm (melatonin timing) and avoid extending wakefulness into the biological night, as sleep debt will disproportionately impair cortical function.