We love our “afternoon siestas”. And at work, afternoons are the least productive time, as we are usually sleepy and lethargic, especially if we have had a heavy lunch.
So why do we feel sleepy in the middle of the day even if we are not in the habit of taking “siestas” ? Some people may mistakenly think that it relates to the consumption of food. In particular, some believe that there is a significant shift in blood flow from the brain to the stomach or gastro-intestinal tract to aid in digestion. Although this sounds plausible, it doesn’t really make that much sense. If this were the case, why wouldn’t we feel just as sleepy after eating a large breakfast or dinner ? The truth is that this “sleepiness” is due to another cause.
It has more to do with the natural timing of an increased propensity towards sleep. There are two phenomena that contribute to this : the “homeostatic” sleep drive and the “circadian rhythm.” The sleep drive is due to the gradual build-up of a chemical within the brain called “adenosine”. The longer a person stays awake, the more “adenosine” accumulates, leading to an increased desire for sleep This reaches its peak right before bedtime, but is also higher in the afternoon compared to the morning.
The second process that contributes indirectly to sleepiness is the “circadian rhythm”. This is actually the pattern of an “alerting signal”. It increases throughout the day to keep us awake and counteracts the increasing levels of “adenosine”. There is a dip in this pattern in the early afternoon, typically seven-nine hours after waking up. When the “alerting signal” dips, the underlying sleepiness shows itself, and we feel sleepy.
Fortunately, if you tough it out, this period will pass and you will find that you feel more alert again in a matter of hours.