It could be their breaktimes, but if supposed to be working, well ......

It seems a lot of those pictures are from daytime hours so I can imagine night shifts play havoc with their sleep patterns but no excuse for daytime naps if they are on duty and not on their breaks.

When I was a nurse night shifts were a killer and it was very hard to stay awake unless busy so I offered to do the mundane jobs to stay awake.