Are you sure that OS still operate when you sleep?, maybe is
hibernating firefox wen you left it alone.
You could have the script write to a server log, or better ... some
sort of clock/counter handled by js, to check if it is running all the
time.
Its the script doing something could be interesting to comment here?,
withouth more information, only people with the exactly the same
problem [solved] can give you useful feedback.
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ℱin del ℳensaje.