Sleep and caffeine
Getting up in the morning and feeling that desperate need for coffee to get started with the day? We all know about the sleep-suppressing effects of caffeine (a substance of coffee), but how does it work? How come caffeine can keep us awake when we would otherwise fall asleep?
Enough with the questions already! What’s the answer? Well to understand how caffeine interferes with sleep or sleepiness, we need to know how sleep works. In a previous discussion I already talked about Borbély’s two-process model, i.e. the homeostatic and the circadian process that regulates sleep. The circadian process regulates the cyclical secretion of hormones e.g. melatonin, and the homeostatic process is the process referring to the increase of sleep pressure with increased wakefulness and the dissipitation of sleep pressure during sleep.
Adenosine is a neuromodulator (not to be confused with a neurotransmitter) that follows the same course as the homeostatic process, i.e. build up of adenosine during wakefulness and decrease of adenosine during sleep. Adenosine binds to adenosine receptors resulting in an inhibition of neuronal activity. So during prolonged wakefulness inhibition will become more prominent due to increased adenosine concentration and will eventually promote sleep. Caffeine however, is an antagonist that binds to the same receptor as adenosine, and prevents adenosine’s inhibitory action. A picture from palobiofarma illustrates nicely how caffeine interacts with the adenosine A2a receptor and prevents adenosine from binding to the receptor, resulting in a behavioral state of wakefulness/activity.
Animal studies have shown additional effects of caffeine, aside from its antagonistic function of the adenosine receptor, but these effects however occur in larger quantities than what we usually consume. For more information about these additional effects and a comprehensive review on the effects of caffeine on behavior, see the reference below (not freely available).
Nehlig, A., Daval, J. L., & Debry, G. (1992). Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects Brain Research Reviews, 17(2), 139–170.