Many lawyers and law students deal with stressful situations by self-medicating with alcohol or other drugs - and at a much higher rate than the national average.
Substance abuse, exacerbated by cultural, law-firm acceptance and overall stress, can lead to poor mental health. And while, granted, every large law firm has a different culture, one of the more common cultures is toxic and accommodates or encourages poor life choices.
Obviously EVERY big law attorney doesn't struggle with addiction or mental health, but it is a sizable number (about 33% according to some studies).
N.Y. Times article if you're interested - link