29 CFR 785.21: Less than 24-hour duty.

An employee who is required to be on duty for less than 24 hours is working even though he is permitted to sleep or engage in other personal activities when not busy. A telephone operator, for example, who is required to be on duty for specified hours is working even though she is permitted to sleep when not busy answering calls. It makes no difference that she is furnished facilities for sleeping. Her time is given to her employer. She is required to be on duty and the time is worktime. (Central Mo. Telephone Co. v. Conwell, 170 F. 2d 641 (C.A. 8, 1948); Strand v. Garden Valley Telephone Co., 51 F. Supp. 898 (D. Minn. 1943); Whitsitt v. Enid Ice & Fuel Co., 2 W. H. Cases 584; 6 Labor Cases para. 61,226 (W.D. Okla. 1942).)