A new study using twitter to evaluate mood over the course of the day reveals that people’s mood rises in the morning, lowers in the afternoon, then rises again in the evening. Hmmm. Where have I seen that curve before?
This is exactly the sleepiness/wakefulness curve we highlighted in The Promise of Sleep. Yet more evidence that the amount of “energy” you have, your amount of wakefulness, is directly tied to your mood. And the reason why coffee’s greatest effect may be in its mood improvement, rather than wakefulness. Here is another recent study along these lines, showing that women who drink coffee are less depressed than those who don’t.
By the way, I don’t think that scientists really understand completely how caffeine works. The current theory is that blocks adenosine receptors in the brain. Adenosine is left over after ATP is used as a cellular energy source, so a rise in adenosine would signal that the body needs to replenish and trigger fatigue. Blocking that signal is thought to block the fatigue. I don’t really buy that is all that is happening. My guess is that there are some direct neural effects.