You might have experienced this quite a lot, especially on Mondays. You wake up to the alarm and find yourself in an urge to snooze it for a few minutes and return to your Pandora to stay there for one more hour. Why this happens? Why does a human have to battle with himself every morning?
Well, the answer to all the above questions is fairly simple. It’s your neurotransmitters. Particularly the one neurotransmitter that promotes sleep—Adenosine. This neurotransmitter is produced when we are awake, then it binds with its receptors to inhibit certain neurons to make you feel tired and drowsy. With this in play and due to the effects of hormones like Melatonin, you gradually fall asleep.
How Adenosine works?
Adenosine is produced by the breakdown of the ATP molecules when we do rigorous physical work or use our brain intensively. As the brain demands more energy, ATP breakdown happens and more energy is produced along with a proportional accumulation of Adenosine (from the breakdown of ADP and AMP).
Now this accumulated Adenosine finds the Adenosine receptors (A1, A2a, A2b, A3) and binds with them. When bound, the receptors (primarily A1) inhibit the brains activity in being awake and promote sleep by relaxing the muscles gradually.
When you go through the different phases of sleep, the Adenosine that is accumulated in the brain gradually metabolizes and slowly let the brain and body to be awaken by the morning.
So if Adenosine is reduced when we wake up, why we still feel the need to return to sleep?
Well, if you have understood how Adenosine works and have a good understanding of the sleep cycles, then it is much simpler to explore this question. The reason why we still want to hug our pillow even after we wake up is that there is a sudden accumulation of Adenosine when we are awaken from our sleep. This eventually will make us feel drowsy and force us to go back to bed. This sudden accumulation of Adenosine is due to the sudden ATP breakdown. So when we wake up, the body requires a substantial amount of energy to come out of sleep. This sudden requirement of energy uses up all the ATP reserves in the brain (glycogen), resulting in the rapid accumulation of the Adenosine from the ATP breakdown. This in-turn induces sleep again (NREM) so that the body can relax and refill the ATP reserves gradually. It usually takes from a few minutes to two or three hours, which explains our morning drowsiness. There is also a supporting theory where the Adenosine receptor like A2a produces the opposite effect, which encourages sleep by exciting the neurons that are inhibited by the A1 receptors. Hence, the overall effect is putting you to sleep again.
Coffee vs Adenosine
Caffeine binds with these Adenosine receptors better and acts as a blocker, which blocks the ‘go back to sleep’ excitation when you wake up. As caffeine is an antagonist for all types of Adenosine receptors, coffee lovers tend to wake up easily than the rest of the people.
Dreams for the win
Besides the above physiological reasons, I believe the psychology during sleep plays an equal role too. As I explore dreams, I hate when someone tries to wake me up during my REM sleep. Though REM sleep rarely occurs during the waking time, it is worth going back to bed to explore the interrupted dream.