29 CFR 785.11: General.

Work not requested but suffered or permitted is work time. For example, an employee may voluntarily continue to work at the end of the shift. He may be a pieceworker, he may desire to finish an assigned task or he may wish to correct errors, paste work tickets, prepare time reports or other records. The reason is immaterial. The employer knows or has reason to believe that he is continuing to work and the time is working time. (Handler v. Thrasher, 191, F. 2d 120 (C.A. 10, 1951); Republican Publishing Co. v. American Newspaper Guild, 172 F. 2d 943 (C.A. 1, 1949; Kappler v. Republic Pictures Corp., 59 F. Supp. 112 (S.D. Iowa 1945), aff'd 151 F. 2d 543 (C.A. 8, 1945); 327 U.S. 757 (1946); Hogue v. National Automotive Parts Ass'n. 87 F. Supp. 816 (E.D. Mich. 1949); Barker v. Georgia Power & Light Co., 2 W.H. Cases 486; 5 CCH Labor Cases, para. 61,095 (M.D. Ga. 1942); Steger v. Beard & Stone Electric Co., Inc., 1 W.H. Cases 593; 4 Labor Cases 60,643 (N.D. Texas, 1941))