If I don’t brush my teeth before bed and I wake up in morning my mouth tastes fine but if I do brush before bed I wake up with gross taste?

Anyone know why? If I brush my teeth before bed and wake up in morning my mouth tastes disgusting!! But if I do not brush my teeth all day then do to bed I wake up and my mouth tastes like nothing, my mouth isn’t dry or gross. Why? It makes me not wanna brush my teeth.
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