Night Light

Night Light refers to a statue of the moon similar to the Eternal Star Statue that is speculated to appear in areas where multiple levels can be accessed around it, with the exception of the first floor of Peach's Castle. The first and so far only sighting of the Night Light was in the Electronic Gaming Monthly test cart of Mario 64. It overlapped with another anomaly, namely the Fourth Floor, somehow appearing in the middle of the floor.

Unlocking Methods
While unlocking methods for Night Light vary, the unlocking method of the EGM test cart was to beat The Starry-Night Skies, which had it's painting on the Fourth Floor instead of the second. The chances of it appearing as well as the unlocking methods are chosen by the AI, meaning the chances of it being able to appear alongside other separate anomalies is unknown.

Night Mode
When you unlock Night Light, the moon statue gets replaced with a navy blue moon featuring white eyes. You collect it like a star, although it does not exit the level. If you go back to the file select, a new button called Night Mode will appear. Pressing this option changes the File Select music to have less instruments and a music box-like instrument. It also forces you to play as Luigi. Playing the game in Night Mode makes all courses night-themed and changes their music like it changes the File Select theme. The castle uses the night time themed textures from the Spaceworld demo.

Theorising
It is speculated that Night Mode was planned alongside the unused multiplayer features, and was supposed to be some sort of New Game Plus/hard mode, similar to Super Mario Bros, or Super Mario World, given how it reskins the levels, and gives you Luigi's signature traits, a higher jump and slippery controls. It is unknown if any anomalies are affected by Night Mode or not, although it is theorized it could have effects on the removed courses, or the Internal Plexus.