I deleted my original post and will add some pertinent information from the Federal Fair Labor Standards Act.
Sleeping Time and Certain Other Activities: An employee who is required to be on duty for less than 24
hours is working even though he/she is permitted to sleep or engage in other personal activities when not busy.
An employee required to be on duty for 24 hours or more may agree with the employer to exclude from hours
worked bona fide regularly scheduled sleeping periods of not more than 8 hours, provided adequate sleeping
facilities are furnished by the employer and the employee can usually enjoy an uninterrupted night's sleep. No
reduction is permitted unless at least 5 hours of sleep is taken.
So basically, it's legal and they're actually being “generous.”