Why did bombur fall fast asleep inside mirkwood?

Bombur fell fast asleep inside Mirkwood due to the enchanted nature of the forest, which caused him to become drowsy and disoriented. The thick, dark atmosphere and the influence of the magical surroundings contributed to his deep slumber, making him unable to stay awake despite the dangers lurking nearby. This moment highlights the forest's treacherous effects on those who traverse it.