Why do people feel sleepy while reading?

Feeling sleepy while reading for many is due to the posture in which they read.While reading a person rarely moves,this lack of physical activity causes decreased blood flow to muscles,which results in accumulation of lactic acid(a product of incomplete combustion in the cells).This lactic acid is a highly reducing or oxygen absorbing agent,which quickly reacts with oxygenated blood.This ensures lack of oxygenated blood flowing to the brain,which makes us feel sleepy.

To tackle this problem we must avoid reading in a single stretch,instead its better to take some breaks and indulge in physical activity at short intervals.