Caffeine blocks adenosine receptors, which is a molecule that makes our bodies tired. This means that when we ingest lots of caffeine, our bodies won't be able to trigger the chemical reactions that send us signals to feel tired or sleepy. This is why many people experience alertness after drinking coffee or tea.Adenosine builds up in our blood throughout the day before increasing at nighttime, which causes melatonin levels to increase and allows us to fall asleep naturally. When caffeine disrupts this process by blocking adenosine receptors, melatonin levels remain low and this causes difficulty falling asleep, insomnia, and sleep deprivation.