Fitbit blog post, "The Real Reason You Feel Sleepy in the Afternoon" by Michele Bender
In short, it's the way we are wired. This is part of a natural cycle of alertness throughout the day. I provided a short excerpt below and you can click on the link above for the full article.
"If you’re longing for a latte by 4:00pm, blame your body clock. “A sense of sleepiness can occur seven to nine hours after our wake up time due the brain’s drop in alertness,” explains Fitbit sleep advisor Allison T. Siebern, PhD, consulting assistant professor at Stanford University Sleep Medicine Center and director of the Sleep Health Integrative Program at the Fayetteville VA Medical Center in North Carolina. Another reason you’re feeling fatigued? Your body builds up adenosine, a chemical that accumulates in your system to cause sleepiness, as the day progresses, says Siebern. In other words, the longer you’re up, the wearier you become."