Reading "Potatoes Not Prozac" led me to do a lot of independent research on dopamine and sleep.

I found a fantastic, easy-to-digest explanation of why we need to sleep. The short version is this:

Adenosine builds up in the bloodstream and in the brain during conscious hours. It is a metabolic byproduct that inhibits dopamine - a neurotransmitter that helps keep us alert and focused. The result is that with a build-up of adenosine in the blood, we feel lethargic and cannot think.

Caffeine mimics adenosine in the brain. It fools the body into releasing dopamine by blocking adenosine receptors. The result is that caffeine makes us feel awake (temporarily). After appx 4 hours, when the caffeine has metabolized, we crash as the unblocked receptors react to the high levels of adenosine in the system.

Caffeine can't replace the need for sleep, it just masks the symptoms of tiredness.


Here is the complete article:
http://www.scq.ubc.ca/eyes-wide-open-dispelling-myths-and-uncovering-truths-behind-caffeine-consumption/

Staying awake

Natural sleep and wake cycles in humans depend largely on the activation of cell membrane adenosine receptors in the striatum of the brain. Adenosine receptors are ubiquitously distributed in the human body with a variety of functions. The natural ligand for these receptors, adenosine, is a metabolic end product of most cells. Throughout the day, adenosine builds up in the circulation. The build up of adenosine in the brain binds and activates the adenosine receptors, which then causes decreased activity of the neurotransmitter dopamine. Dopamine in the brain has a stimulating effect causing improved cognition, motivation, increased movement and awareness. Therefore, by decreasing dopamine activity, the body falls into a state of fatigue indicating the need to sleep. During sleep, circulating adenosine is metabolised and cleared thereby restoring dopamine activity causing the brain to wake up.

Within 45 minutes following caffeine consumption, it is completely absorbed and distributed throughout the body. Circulating caffeine can readily cross the blood-brain barrier. Due to its structural similarity with adenosine, caffeine in the brain can act as an antagonist such that it binds adenosine receptors but do not activate them. Therefore, binding of caffeine reduces adenosine activity. This increases dopamine activity, which is responsible for most of caffeine’s psychostimulating effects. The clearance of caffeine varies among individuals depending on age, health conditions and current medications. In healthy adults the half-life of caffeine is approximately 3-4 hours. The half-life is much longer in patients with liver diseases, in young children and in women who are pregnant or taking oral contraceptives. Since caffeine blocks the binding of adenosine to its receptors but do not induce adenosine clearance, once the circulating caffeine level drops, the individual will feel fatigue and tired again. In other words, caffeine does not eliminate the need to sleep, but simply temporarily mask the sensation of sleepiness. Furthermore, while functioning on caffeine, the adenosine builds up even more in the brain such that when the caffeine effect wears off, the fatigue felt is much stronger than before caffeine consumption. Basically, caffeine can allow the individual to stay focused for a few hours, but the recovery sleep required is often more than just a few hours.

Adenosine build up in the brain also acts on the blood vessels to induce vasodilatation and increase blood flow, which are necessary for deep sleep stages. Needless to say, caffeine will antagonize these effects too preventing deep sleep stages from occurring. This explains why it is difficult to fall asleep or feel refreshed after sleeping while under the influence of caffeine.