When we sleep, we in other words, go to the hibernation mode. We sleep to attain peace and relax after the physically and mentally worked out day we go through.
When we sleep, our mind enters resting mode. We feel relaxed. However sometimes we do dream. Dreams are consequences of the last thoughts we have in the back of our minds. Sometimes, we also dream about things that could even happen for real in future.
We often wake up in between dreams, due to some noise or some break or uneasiness. This could be due to bad dream or maybe some clatter around.
In sleep, we are totally not aware if something happens around. So any sudden noise externally, can wake us up as a instant response.
I hope this answer helps!