29 CFR §553.222

1. (a)Where a public employer elects to pay overtime compensation to employees in fire protection activities and/or law enforcement personnel in accordance with section 7(a)(1) of the Act, the public agency may exclude sleep time from hours worked if all the conditions in § 785.22 of this title are met.
2. (b)Where the employer has elected to use the section 7(k) exemption, sleep time cannot be excluded from the compensable hours of work where
   1. (1)The employee is on a tour of duty of less than 24 hours, which is the general rule applicable to all employees under § 785.21, and
   2. (2)Where the employee is on a tour of duty of exactly 24 hours, which is a departure from the general rules in part 785.
3. (c)Sleep time can be excluded from compensable hours of work, however, in the case of police officers or employees in fire protection activities who are on a tour of duty of more than 24 hours, but only if there is an expressed or implied agreement between the employer and the employees to exclude such time. In the absence of such an agreement, the sleep time is compensable. In no event shall the time excluded as sleep time exceed 8 hours in a 24-hour period. If the sleep time is interrupted by a call to duty, the interruption must be counted as hours worked. If the sleep period is interrupted to such an extent that the employee cannot get a reasonable night's sleep (which, for enforcement purposes means at least 5 hours), the entire time must be counted as hours of work.