Very tired slaves are both less productive and less healthy. The exception is if they are mindbroken, very devoted, or very fearful, in which case they will literally work themselves to death without loss of productivity, if so ordered.
Slaves that are permitted rest, and required to work actively (i.e. servants, students, and sex workers) will voluntarily reduce their own productivity by resting when they get tired, to maintain their health and long-term productivity. Cows notably do not suffer from reduced productivity when resting, since they can be milked just as well in that state.
There is a balance between mandating rest (which reduces productivity) and forcing slaves to work while tired (which also reduces productivity). This balance is different in different jobs, and you might find it worth experimenting. Note that slaves with better living conditions don't require as much time resting, because the rest they do get is higher quality.
Mandatory rest will increase a slave's trust, unconditionally. Permissive rest rules will generally increase both devotion and trust depending on the exact circumstances. Restrictive rest rules will increase the fear of non-devoted slaves, but increase the trust of devoted slaves. Cruel rest rules will build fear rapidly in non-devoted slaves, but degrades devotion in devoted ones; it can be used as a breaking method (or to kill unwanted slaves Gulag-style) but isn't really recommended byond that.
