adverb
BAD : I recently stayed an overnight at your hotel.
GOOD : I recently stayed overnight at your hotel.
BAD : I am writing to complain about my overnight at your hotel.
GOOD : I am writing to complain about the night I spent at your hotel.
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Overnight is used as an adverb and adjective (NOT as a noun):
'I missed the connecting flight and had to stay at the airport overnight.' 'If we travel overnight, we'll be there in time for breakfast.' 'The overnight coach arrives in London at six in the morning.'