hi this is gladmohesh .manipalite ,now in chennai as lecturere.will tell u something that i have observed.first usually students will sleep in the afternoon classes.maybe they need rest after a lunch.i feel that excess o2 is needed for the git after alunch to digest and assimilate the things eaten.second early morning classes they go for a deep sleep .mayb bcoz the previos night they didnt have better sleep and so after a cold water bath they feel sleepy .as body takes time to rejuvenate the energy lost.third ,of the most important i have experienced is if the class room was locked up the night with no air circulation and the next day mornign when 50 -100 students sit together and take all the o2 available co2 build up and so i think most of te students go for a nap even with eyes wide open.above all 4th is that when the physiology lec r prof goes for a continous lec for more than 3o mins without the student understanding what he wanted to know fatigue sets in and he finds the profs lec as a lullaby adn goes for a fine nap.