A more thorough explanation:

Definition: A nightwalker is a person who wanders around suspiciously at night and may disturb the peace. It can also refer to a prostitute who walks the streets at night or a sleepwalker.

• A man was arrested for being a nightwalker and disturbing the peace in the neighborhood.
• The police officer saw a nightwalker on the street and asked him what he was doing out so late.
• She worked as a nightwalker to make ends meet.
• He was a known sleepwalker and would often wake up in strange places.

The first two examples illustrate the historical use of the term to describe someone who was suspiciously wandering around at night. The third example shows the more modern use of the term to describe a prostitute who works at night. The fourth example shows the medical use of the term to describe someone who walks in their sleep.