Laws regarding 24 hour shifts and breaks?

[bronsin]

My partner recently started working at an ambulance company and they've decided to begin 24 hour shifts. He was supposed to start at 11am and work until 11am the next day with a 4 hour block of uninterrupted rest during that time. However, while at work his boss said that the 4 hours of rest would instead be counted as the downtime in between calls. This doesn't seem right to me - working a straight 24 hour shifts with no specific downtime to rest - but I'm not familiar with 24 hour shifts and California's labor laws so I was wondering if anyone happened to know the regulations regarding 24 hour shifts and breaks.

Thanks so much.