I often think of it in terms of productivity. If I have no stress I just meander along and don't seem to accomplish much. If I have too much stress it frazzles my brain and I use up too many resources simply coping with the stress, and productivity goes down. If I have a 'good' amount of stress I feel hyped and driven and can really get in and get a lot done. Unfortunately peak performance is achieved at stress levels very close to the tipping point...