A single 100mg dose of caffeine taken at bedtime reduces low-frequency delta activity (0.75–4.5 Hz) in the sleep EEG during the first non-rapid eye movement sleep episode in healthy young adult males, indicating a specific disruption of deep sleep brainwave patterns without fully replicating natural sleep dynamics.

Caffeine enters the brain and sticks to adenosine receptors, preventing adenosine from slowing down brain cells. Without adenosine’s calming effect, brain cells stay more active and fire out of sync, which stops the deep, slow brainwaves that happen during the first stage of deep sleep. This makes the brain’s electrical activity look more like wakefulness than true deep sleep.