Some cities do not turn off their water pipes at night. In fact, most cities keep their water pipes pressurized at all times to prevent contamination and ensure a reliable flow of water. However, there are a few reasons why some cities might choose to turn off the water pipes at night:

1. Extreme Weather Conditions: In areas with extremely cold temperatures, cities may shut down the water supply to prevent pipes from freezing and bursting. Frozen pipes can cause significant damage to infrastructure, and the repairs can be costly and time-consuming.

2. Water Scarcity: In regions facing water scarcity or drought conditions, cities may restrict water usage during certain times, including nighttime, to conserve water and ensure fair distribution.

3. Maintenance and Repairs: Occasionally, cities may shut off the water supply to perform maintenance work or repairs on the pipes or water treatment facilities. These interruptions are typically scheduled and communicated to residents in advance.

4. Conservation Efforts: As a part of water conservation strategies, some cities may choose to reduce water pressure or turn off the water supply during certain times, including at night, to minimize water consumption.

5. Security Measures: In rare instances, cities may shut down the water supply for security reasons, such as during civil unrest or public emergencies, to prevent potential tampering with the water system.

It's important to note that if a city does turn off the water supply, residents are typically notified in advance so they can take necessary precautions. Additionally, many buildings have backup water storage tanks or reservoirs to maintain water flow during such interruptions.