It's likely because the body is too much to process purified/processed carbohydrates such as white rice and sugar. Food items such as these, especially sugar, produce instant energy but only a short time then immediately drop so that the body feels weak. In addition, there's the possibility that the body is sensitive to carbohydrates.
To fix this, try eating more complex carbohydrates interspersed with protein and fiber from vegetables and fruits. Don't forget to drink water.
For certain people, sleepy after lunch could occur because the body too much expends energy to digest food. Therefore, eat always in ample portions.
You may also like:
10-Day Green Smoothie Cleanse: Lose Up to 15 Pounds in 10 Days!
Dr. A's Habits of Health: The path to permanent Weight Control and Optimal Health
Presentation Zen: Simple Ideas on Presentation Design and Delivery (2nd Edition) (Voices That Matter)
What I Talk About When I Talk About Running (Vintage International)
School for Startups: The Breakthrough Course for Guaranteeing Small Business Success in 90 Days or Less