Falling Asleep at Work

I work at Wal Mart as a Courtesy Associate so I push buggies all day and I have come close to falling asleep,because if I am working morning shift,at like seven or eight in the morning it is slow,or if I am on second shift,at like eleven pm it is slow so I will sit down and wait for something to happen,and they also make me take two fifteen minutes and when I am sitting down sometimes I will almost fall asleep and when I am tired I will often get a soda from the drink machine to help me wake myself up,and I have never had to be woken up by a coworker especially since there is only usually one cart pusher at a time,so there is usually no one else in the parking lot except customers.