Can Stress Cause Hair Loss?

I am a 34 year old man and I started losing my hair about two years ago and it hasn't stopped. My hair is thining and it falls at all times. I have a great diet, I exercise daily, I don't smoke or even drink. One of the things that changed when I noticed it was falling out was when I started my MBA and I was under a lot of pressure and stress. Before I started school I had a lot of thick hair. Could stress be the cause of my hair loss?