Be open with your management if you feel overworked. They might not know how you feel.. Keep that line of communication open. Good managers will understand. If its really bad or you found a better job, then leave.
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Nobody gets paid more by getting a raise nowadays, they've already got you under contract and they don't want to pay you more. If you negotiate well you can get a 15% increase in salary when you switch jobs. You'll never get that if you ask for a raise. So it's the smart decision to leave.
@Lynx122 It all depends on what you do and what you are worth, but if you’re thinking of jumping ship anyway to a job at another company there is no harm and taking the risk to claim a larger piece of the pie. That’s all part of the equation. When you make demands for more pay, you have to be willing to walk away or else there is no weight behind your words.