My understanding is that sleep deprivation significantly impacts mental alertness. It could be considered fatal in that one is more likely to die in an accident related to their mind failing to function properly.

IIRC, some law enforcement groups theorized that many accidents were caused not just by drugs/alcohol but rather by the decline in attention, alertness and functioning of the brain due to lack of sleep.

Speaking of which, my productivity in the classroom seems to be heavily influenced by sleep or lack thereof.