Apostrophe + S is used to show possession in English. In the case of the idiom you cited, you can think of it as the "sleep" belonging to the "night."

In the phrase "a good night's sleep," the 's is a possessive form indicating that the sleep "belongs to" or is associated with the night. This type of construction is common in English to show that one thing is a characteristic or result of another. Explanation: * Possessive Form: The 's in "night's" is a possessive form. It shows that the sleep is associated with the night. * Meaning: "A good night's sleep" means a good quality of sleep that happens during the night. This construction is used to indicate that something is related to or comes from something else. Examples: * A day's work: The work done in a day. * A moment's thought: The thought that happens in a moment. * A week's vacation: The vacation that lasts for a week. Using the possessive form this way is a common and natural part of English grammar to express relationships between things.