Turning down an extra shift doesn’t make you lazy. It means you know what your time is worth.

Every available hour gets treated as an hour you should be selling, so turning down a shift reads as not wanting it enough, even to you. Hours are the one thing you cannot make more of, though. Extra money is a good trade only when what it costs you is worth less than what it pays, and that depends entirely on the week.

Do the arithmetic before answering, and count the parts that are not money: sleep, studying, the evening you had plans for. Sometimes yes is obviously right, especially when you need it. What matters is that it was a decision rather than a reflex.