Which shift is better for police officers, 8, 10, or 12

Im to write an essay for my introduction to policing class. 
For the purpose of this question, you are in charge of a 450+ officer department and your job is to set the schedules.  Will you use 8, 10, 11, or 12 hour shifts and why?  Remember to include information about quality of life, retention of officers, fatigue and sleep, errors on the job, job performance and impact on overtime which impacts the budget.