When he worked at a Southern California law firm, Luke Cocalis had one cellphone, a Samsung Galaxy. But using the single device for both personal and professional activities became stressful in ways he hadn't foreseen.

"I would pick up the phone for something else and I would see a work email, and I would feel the temptation to get involved," says the 28-year-old Encinitas, Calif., resident. Some of the work emails, he says, were the kind that would "ruin your night."

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