Simplify the following text for a Grade 5 native English-speaking person.
Original: Shift work is a working practice which is designed to use all 24 hours of a day , and not only the standard working day hours .
Simplified: where a firefighter will work 24 hours on , 24 off , 24 on , 24 off , 24 on , 96 hours ( 4 days ) off .