Your 24-hour circadian rhythm is the primary factor that determines your sleep and wake cycle. Alongside your circadian rhythm is a complementary sleep pressure system in your brain. This system is especially susceptible to being disrupted by caffeine.
If you want to keep your sleep cycles on track, it’s a good idea to understand how your genes, gender, diet, and dosage all impact how your body processes 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 wipes the slate, resetting your adenosine levels.
Caffeine has a unique ability to disrupt this signal by transitioning from your blood into your brain—crossing a difficult border known as the blood-brain barrier.
Without caffeine, adenosine binds to receptors that send sleepiness signals throughout your body. However, if caffeine enters the brain, it fits in the same receptors and 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 experience a crash because all your adenosine has been building up and binds all at once. 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.
For example, if you have a cup of coffee at 3 p.m., by 8 p.m. you have only metabolized half of that caffeine—the other half is still in your system.
However, half the population has a gene that leads to slower processing of caffeine, meaning the half life is even longer. Caffeine is metabolized by a particular enzyme in your liver, determined by your genes. Fifty percent of the population has a variant in this gene—CYP1A2—that makes them a slow vs. fast metabolizer. You can find this data through companies like 23andMe.
Other factors, such as hormones, may also impact how you break down caffeine. Women using hormonal 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 and may be evident in your RHR pattern as you sleep.
You might also see these patterns on days when caffeine is still in your system at bedtime:
Try observing how caffeine impacts you and keep in mind that soda, tea, medication, and even chocolate can contain enough caffeine to disrupt your sleep.