Circadian rhythm, Melatonin, Adenosine, Caffeine and Sleep

This piece will be my summary of the first two chapters of a wonderful book called “Why we sleep”. I feel a mixed feeling of concern and excitement after reading these two chapters, and would like to share what I learned. The two chapters use science to describe the two primary factors influencing our sleep and the consequences of our normal behavior nowadays, including the effect of caffeine. Knowing these consequences helps a person make better decisions to improve his/her sleep and health.

I highly recommend the book to anyone who cares about sleep and his or her health. All the good findings are from the book. All the clumsy explanations and communication are mine.

Melatonin & circadian rhythm

Everyone has a 24-hour rhythm called a circadian rhythm. The internal 24-hour clock in our brain communicates the circadian rhythm to every area of the brain and part of the body. When the sun sets, our body starts to release a chemical called Melatonin. Melatonin signals to our body that “it’s getting dark, it’s getting dark” and that the time for sleep is close. As we sleep, the chemical starts to wear off. As soon as the sun rises and interacts with our eyes, our brain knows that it’s time to stop pumping Melatonin into our bloodstream. Once the chemical stops circulating, the brain and body know that it’s time to wake up. The rhythm continues in the same way every day regardless of our lifestyle.

It’s worth noting that Melatonin has little effect on why we feel sleepy. It is just a signaling chemical released by our body to trigger a certain action. In this case, it’s a) knowing that it’s dark b) getting up now.

Adenosine and the sleep process

As we are awake, our body constantly produces a chemical called Adenosine. The more Adenosine is accumulated, the sleepier we feel. It is because the concentration of Adenosine will trigger the sleep-inducing part of our brain and mute the wake-promoting region. The production of Adenosine happens only when we are awake and stops when we are sleeping.

The diagram below will explain why the urge to sleep is the biggest at 11pm or midnight. The blue line is our sleep process which represents the level of Adenosine. It rises from 7am to 11pm and decreases when we are asleep. The black line represents our circadian process. It doesn’t change because of our lifestyle. On the other hand, the blue line can certainly does

Sleep cycle original

What will happen if we pull an all-nighter?

Sleep deprivation

As we stay awake during the night, the level of Adenosine continues to rise. Around 4-5 am in the morning, we will feel particularly sleepy since the level of Adenosine is the highest at that moment so far (the orange line). We will feel better in the morning, especially at the peak of our circadian rhythm. However, the level of Adenosine continues to accumulate and later in the evening, we will be hit by a wave of sleepiness that is even harder to resist. To remove the extra sleep pressure from an all-nighter, we will have to sleep longer in the morning. However, who can have the luxury of sleeping till 10am in the morning during the weekdays? As a result, we become sleep-deprived to some extent. There is always a residue of Adenosine from the previous day in our body and it will keep us sleepy, unproductive and listless.

The same happens when we party, go out or binge-watch series late at night. Instead of going to bed around 10-11 pm, we stay up late till 2-3am. Our body has only 4 hours of sleep. There will be plenty of Adenosine left to be carried over to the following day. If the behavior repeats, it will accumulate and we will constantly feel lethargic and sleepy. After a while, even longer sleeps on the weekends may not be enough to remove all the lingering Adenosine. And would you want to sleep in the whole weekends when the weather is nice outside? With family obligations, will there be enough time for sleep on the weekends?

Additionally, our sleep process and circadian rhythm can help explain why we feel easier to sleep when travelling Westward than when travelling Eastward.

Travel effect on sleep

When we travel East, we are forced to sleep earlier (the orange line) than we normally do. On the other hand, as we travel Westward and are tied up with business or social obligations, we would tend to sleep later when we normally do (the purple line).


To fight back against the urge to sleep, we tend to rely on caffeine. Caffeine does make us feel more awake and less prone to falling asleep. How does it do that?

Caffeine blocks Adenosine from interacting with the receptors in our brain, an interaction that would cause sleep-inducing effects. While being blocked from caffeine, the sleep-inducing chemical will keep increasing while we are awake. On other hand, caffeine is worn off gradually by our body. Eventually, caffeine in our body will disappear and Adenosine will be free to interact with the brain’s receptors, this time in an accumulated amount.

Caffeine effect

If we drink coffee late at night to stay awake and our body doesn’t remove caffeine fast enough, we can stay up later. Once the caffeine disappears, Adenosine in an increased quantity will attack our receptors and the urge to sleep is even bigger than it normally is (the purple line)

Caffeine effect_2

According to the book “Why we sleep”, it takes our body on average five to seven hours to remove 50% of the caffeine consumed, meaning that if a person has a cup of coffee at 8pm, it’s like that 50% of the caffeine is still in that person’s body by 1am. Of course, each body is different in how fast it can wear the caffeine off. That’s why some people don’t seem to be much affected by caffeine while others are more prone to the chemical’s effect. Plus, the older we are, the more slowly the caffeine-removing process takes place.

As a result, keep in mind the effect of Caffeine before you decide to sip that hot and delicious cup of coffee or tea at night.

Consequences of sleep deprivation

  • Diminished immune system
  • Higher risk of cancer
  • Higher risk of Alzheimer
  • Higher exposure to diabetes
  • You feel hungry despite being full. Hence, you’ll be more susceptible to gaining weights


H. Keong. (2015). Vulnerability to Sleep Deprivation: A Drift Diffusion Model Perspective.

M. Walker. (2017). Why We Sleep: Unlocking the Power of Sleep and Dreams

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