Wikipedia: Anecdotes report that stress, both chronic and acute, may induce achromotrichia earlier in individuals than it otherwise would have. Proponents point to survivors of disasters, such as Titanic survivor Harold Bride, or high-level politicians such as Bill Clinton or Barack Obama. There is some evidence for chronic stress causing premature achromotrichia, but no definite link has been established. It is known that the stress hormone cortisol accumulates in human hair over time, but whether this has any effect on hair color has not yet been resolved.
Google News: Hyperactivation of sympathetic nerves drives depletion of melanocyte stem cells.