In the night, it is a lot cooler as

In the night, it is a lot cooler as the sun goes to sleep. So the air at the upper slope of the mountain cools off very quickly and becomes dense. A high pressure is created. At this time, the air at the valley floor is a lot warmer (low pressure) and is forced to give way to colder air moving down the slope towards the valley floor. This is called mountain breeze, and it is a lot common in the colder months when there is less warming from the sun.