Your Circadian Rhythm (or 24-hr body clock) is the primary factor that determines your sleep and wake cycle.
In addition to this is a sleep pressure system in your brain. This system is particularly susceptible to the disruption of caffeine.
Caffeine tricks your brain into thinking it’s not tired by blocking your sleepiness signal, adenosine. Adenosine is a brain molecule (neurotransmitter) that helps register elapsed time.
The longer you are awake, the more adenosine will accumulate in your brain, creating pressure to sleep. Sleep then clears the slate, resetting your adenosine levels for the next cycle.
Caffeine has a unique ability to disrupt this signal by crossing the blood-brain barrier. When caffeine enters the brain, it blocks adenosine from binding. When adenosine receptors can’t fire, your brain falsely registers that it has been awake for a shorter period of time, making you feel more alert.
When caffeine leaves your system you consequently experience a crash. This experience can vary by person, as caffeine is cleared from your system at different rates.
Caffeine doesn’t last forever—it has a half-life of 5 to 7 hours, which means it takes that many hours to break down half of the caffeine in your system.
So, if you drink a cup of coffee at 2 p.m., by 7 p.m. you have only metabolised half of that caffeine—the other half is still in your system. Half of the population may have a gene that leads to slower processing of caffeine, meaning the half-life is even longer.
Caffeine is metabolised by a particular enzyme in your liver, determined by your genes. Fifty percent of the population may have a variant in this gene that makes them a slow vs. fast metaboliser. Other factors, such as hormones, may also impact how you break down caffeine. Women using birth control, for example, have an extended half-life for caffeine, meaning it stays in their system for much longer.
When caffeine is in your system, it activates your nervous system and increases your resting heart rate (RHR). This can interfere with your ability to wind down before bed.
Try observing how caffeine impacts you and keep in mind that soft drinks, tea, green tea, medication, and even chocolate can contain enough caffeine to disrupt your sleep.