While comparable US employees reporting elsewhere are not, these are the violations:
National-origin discrimination — Title VII, 42 U.S.C. § 2000e-2
If the different scheduling is because of national origin.
Adverse working conditions for U.S. workers — INA § 212(n), 8 U.S.C. § 1182(n)
Particularly relevant if other employees doing comparable work receive better hours or working conditions.
Overtime violations — FLSA, 29 U.S.C. § 207
If the overnight schedule results in more than 40 hours per week without required overtime pay.