It is called “The First Night” effect

During the first night in unfamiliar surroundings the left side of the brain stays awake and the right ear, which is connected to the left side of the brain, remains more alert to unusual sounds.

Sleep researchers discovered the “first-night effect” decades ago, when they began studying people in sleep labs. The first night in a lab, a person’s sleep is usually so bad that researchers simply toss out any data they collect.