In nearly all major cities, the metro system closes during the darkest hours of the night. Though that time is oft en used for cleaning and repairs, this shutting down may be an unconscious nod to the night’s unique rhythms—as if the tracks themselves might change in the midnight hours, the day’s trains not designed to traverse them. Of course, there are taxis and night buses, but when traveling in the dark, walking should be our preferred mode of transportation.

Nightwalking—choosing to perambulate after businesses have shut down and most people are asleep—is rebellious. It serves few, if any, obvious purposes, and that deliberate purposelessness is precisely what makes it seem unacceptable. In a world dominated by crisp images, products designed for ultra-specific uses, over...