Sleep and Caffeine

Caffeine has a half-life of roughly 5-6 hours in most adults, meaning half of a 3pm coffee's caffeine is still active in your system at 9pm, and a quarter remains at 3am. It works by blocking adenosine, the chemical that builds up sleep pressure through the day — so it doesn't just keep you awake, it actively reduces how sleepy you feel.

Even if caffeine doesn't stop you falling asleep, research shows it still reduces total deep sleep and increases night-time awakenings — you can drink coffee, fall asleep fine, and still get measurably worse sleep quality without realising why.

A practical cutoff is 8-10 hours before bedtime for most people — so a 10pm bedtime means no caffeine after roughly noon to 2pm. Sensitivity varies significantly between individuals due to genetics, so some people can tolerate an afternoon coffee fine while others can't.

Watch for hidden caffeine sources beyond coffee — tea, energy drinks, some soft drinks, chocolate, and certain pain relief medications all contain meaningful amounts.