Can it cause damage to a computer if it is left on for a long period - nearly 30 hours?

It's currently been on for 1 day, 5 hours and 10 minutes. I've been trying to get my coursework done, and yes, I'm knackered.
Update: Just to clarify, I don't mean in stand-by mode, I mean actually on and in use for the whole 30 hours, PC and monitor.
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