There are three likely explanations, based on recent research.
First, what do you eat and drink?
A lunch high in carbohydrates, especially if no protein is eaten along with bread and milk, is likely to produce calm to the point of sleepiness, because of the complex role of carbohydrates in shifting the balance of neurotransmitters in the brain.
Conversely, some protein contributes to alertness. A three-martini lunch also induces sleepiness.
Second, as siesta-loving societies have long recognized, sleep researchers have found that there is an inborn readiness in human beings to take a midafternoon nap.
Volunteers isolated from all evidence of day and night and left to set their own sleep patterns tended to settle into one long sleep session and a session of 1 or 2 hours about 12 hours after the middle of the main period.
Third, you may be sleep deprived. While the average adult needs 8 to 9 hours of sleep a night, most get only 7, and many get 6 or less during the workweek. People permitted to sleep as much as they choose sleep 10.3 hours out of every 24, just as monkeys and apes do.
An afternoon nap may be just what nature ordered to restore alertness.