In after the 1991 Gulf War, PTSD rates spiked beyond rates ever seen in the US Army. A study commissioned to examine the effects of PTSD found that this long-term stress was actually causing the hippocampus (the short-to-med/long term memory module of the brain, amongst other things) to atrophy. This confirmed a similar effect they had found with mice a few years earlier.

The next push is to replicate the more positive result of environment enrichment, which has been shown in mice but not people. Basically, environment enrichment has been shown to help facilitate LTP (long-term potentiation — an extreme period of neurogenosis) and makes the hippocampus less prone to stress-induced atrophy.

I've personally been a few very high stress work environments, but I use this knowledge to help convince myself to relax productively.