29 CFR § 525.6 - Compensable time.

Individuals employed subject to this part must be compensated for all hours worked. Compensable time includes not only those hours during which the individual is actually performing productive work but also includes those hours when no work is performed but the individual is required by the employer to remain available for the next assignment. However, where the individual is completely relieved from duty and is not required to remain available for the next assignment, such time will not be considered compensable time. For example, an individual employed by a rehabilitation facility would not be engaged in a compensable activity where such individual is completely relieved from duty but is provided therapy or the opportunity to participate in an alternative program or activity in the facility not involving work and not directly related to the worker's job (e.g., self-help skills training, recreation, job seeking skills training, independent living skills, or adult basic education). The burden of establishing that such hours are not compensable rests with the facility and such hours must be clearly distinguishable from compensable hours. (For further information on compensable time in general under FLSA, see part 785 of this title.)