When I first received this advice, I didn't follow it. I spent most of film school sitting down: watching the classics, writing, researching, editing, and rewriting. While there is an inside industry joke about writers being overweight and unattractive, there is some truth in at least the overweight part.
Personally, I gained 90 pounds while attending college. A combination of hours of sitting, plus junk food and Rockstar energy drinks, used to stay awake to make sure my short script was completed in two days. Add stress in the mix (Hurricanes Ivan and Katrina, lack of funds to attend school, family issues, working extra jobs, but no internships) and you have a recipe for disaster. While I did go to the gym, cycled, and walked, I did not do enough to reverse the hours spent sitting on my ass putting as much sugar as possible in my system. I wanted an A too badly.
There is a joke at USC Cinema. While there, you will either be addicted to coffee or cigarettes. I was addicted to coffee, but the number of smoking students was staggering. I also minored in the Roski School of Fine Arts. Given how much time I spent at both schools, I've probably cut my lifespan by a few years from all the second-hand smoke.
My addiction to coffee and my long-distance relationship led to many sleepless nights. Sleep is uberimportant. Neglecting sleep will also lead to health problems. I still don't follow this like I should, but I'm getting better at it.
I was never a large drinker (despite being part-Irish), so I don't worry about drinking too much. I usually only stick to wine or mix drinks. I had to learn to drink socially to be more effective at networking. I've been drunk a grand total of three times. It's not a good feeling, and you may have a problem if you find yourself at the end of a bottle more days than not.
Learn from my mistake: take care of yourself. You will feel better and be more productive. For example, I have lost 30 pounds and try to squeeze in a little more activity in my day before sitting down to write. I eat better, even though I still have a habit of eating in front of a screen. I also stretch and walk around between pages. I go to the doctor now, mostly out of the motivation I would want to have children in the very near future. I still have a long way to go, but I am now making progress.