the.com/sleepiness
Your brain quietly filing a resignation letter while you pretend to keep reading.
means The growing pressure to fall asleep, driven by how long you've been awake and your internal clock.
from From Old English 'slǣp', sharing roots with 'slack' — the body literally going slack as alertness loses the argument.
Chemical culpritAdenosine builds all day; caffeine just blocks it.
Two systemsSleep pressure plus your clock decide the timing.
MicrosleepsBrains nap for seconds while eyes stay open.