Sleep research has identified generally times of the day when adults are most likely to want to sleep: 3-7AM and 1-3PM secondarily. In other words, feeling sleepy in the afternoon is normal. Perhaps it is industrial society that has dictated what are supposed to be our "normal" sleep pattern. I noticed when I was working that at 2:30 PM every day, I had a period of overwhelming sleepiness. It went away spontaneously. The intensity of this feeling was overwhelming the past 5 years. One of my physician colleagues who works 24 shifts always tried for a brief afternoon nap, as that improved her focus at other times.
.When I was in college, as my day permitted, I would take a nap as soon as I got home at 5PM and woke up at 6PM, ate dinner, and went back to studying until 11PM-midnight. It was a pattern that worked spectacularly well in terms of grades.
Sleep can be amazingly restorative. It has been years since I even caught a cold, but if I get sick even with a cold, I do much better if I can send myself to bed for extra recovery time.
Here is a link to an interesting and informative research group on sleep:
Body Clock & Sleep - National Sleep Foundation
It's ok to be sleepy in the afternoon. It doesn't mean you are ill.