NiGHTS into Dreams
"Nightopians are carefree, happy inhabitants of Nightopia. They’re born every time balls of light (called Ideya) collide. The sole reason for their existence is to have fun. Because they are incapable of doing anything else, they have no need to work together or to cooperate towards a common goal. As a result, they have no leader, and no language. What they do have, is the ability to see the consciousness of each visitor (dreamers from our world), through Ideya. When visitors arrive, the Nightopians can see their consciousness and recreate the happy world that is hidden deep inside. They’re why Nightopians, who don’t have the capacity for intelligent thought, can sing, dance, and even go fishing."
- As stated from the original NiGHTS into Dreams web page, no longer in existence.
Nightopians were Sonic Team's first Announced "Artificial-life" (A-Life) The game keeps track of the moods of the Nightopians (harming them will displease them, for example), and the game features music engine that changed the background music (BGM) depending on the mood of the Nightopians in that specific level changing tempo, pitch, and melody. When two Nightopians touch, there is a chance that an egg will appear. By paralooping a Nightopian, you will kill it, and lower the mood of every other Nightopian in the level, and drill dashing near one will cause it to be afraid of you, and will lower that specific Nightopian's mood. Nightopians will either follow you, run away from you, or be completely indifferent to your presence, based on their mood. If one is skilled enough, and has the patience, they can lure any two Nightopians close enough, and breed them. After a few playthroughs of the same level, Nightopians and Mepians will disappear, with the oldest ones being the first to go, as to make it easier for the hardware to load the level and keep a decent framerate. Nightopians share a similar role with the Chao in the Sonic the Hedgehog series of video games and other media. This similarity is especially apparent in Sonic Adventure, Sonic Adventure 2, Sonic Adventure DX Directors Cut, and Sonic Adventure 2 Battle, where the player raises, feeds, and breeds Chao in different Chao gardens.
Mepians, in NiGHTS into Dreams are created by grabbing a Nightmaren minion and releasing them before paralooping them. Releasing them will cause the nightmaren to be thrown off in a ball and bounce around the level a few times. In order to make a Mepian, the nightmaren must accidentally collide with a Nightopian. Upon such an event, an egg will be left behind and out from it a Mepian will be born.  Since they were created by you, upon hatching, Mepians will automatically have a positive mood, and lowering their mood is harder to do than lowering a Nightopian's mood. It is a requirement to have Mepians in your game in order to see the King Nightopian.
Through extensive breeding of Nightopians and Mepians a King Pian, or Superpian can be created. This King Pian can be spotted in one of the dream levels by the tower he starts to build. The King Pian, as well as his tower, varies on how well you treat you Nightopians and Mepians alike. Unknown to most however, is just how easy it is to create a king. Kings are created by being a child Pian of two parent Mepians with different parts, and since parts are inherited by children at random, it's entirely possible to create a king by luring two Mepians close enough to breed.
Christmas NiGHTS into Dreams
Nightopians act the same in Christmas Nights into Dreams as they do in NiGHTS into Dreams. However, their appearance changes for the holidays to sport Red and white hats and matching outfits.
The NiGHTS into Dreams demo disc, also known as Christmas Nights into Dreams, features an unlockable feature known as the Nightopian Collection where you can view your Nightopian information stored in a saved game file. Information displayed is overall happiness of each dream level as well as a graphic for Nightopian and Mepian. This feature is helpful when trying to create the Super Pian.
NiGHTS: Journey of Dreams
Nights: Journey of Dreams allows Nightopians to be collected for the Nightopian Garden by paralooping them. After being paralooped, the nightopians will show up in your Nightopian Garden where they'll sing, play, and add to the garden depending on which levels you play the most.
Nightopians appear in both NiGHTS into Dreams and NiGHTS Journey of dreams, as well as Christmas NiGHTS into dreams, in the NiGHTS into Dreams Archie Comics, two Nightopians called Napp and Snuze are major characters who helped NiGHTS after he started his uprising, Nightopians and other NiGHTS characters also appeared in Game Gag 1P Comic Sega-hen, a 1997 Japanese Gag Comic from the Gēmesuto Komikkusu series dedicated to SEGA games.
Outside of NiGHTS, a Nightopian appeared in Sonic Adventure and Sonic Adventure DX as a card, in Shenmue as a capsule toy and in ChuChu Rockets! as an special costume, some Nightopians can be spotted in the NiGHTS tables from Sonic Pinball Party, in Sega SuperStars Tennis, they appear during NiGHTS all star move and on the NiGHTS tennis stage, in Sonic and All Star Racing Transformed a Nightopian appears driving NiGHTS shapeshifted as a car and a lot of Nightopians can be seen in the Dream Valley circuit , in Sonic Lost World, Sonic has to save some Nightopians who were made prisioners in the Nightmare Zone DLC stage, they appear at the very end of the level and in Sonic Runners, a Nightopian and a Nightopian wearing a christmas costume appeared as an unlockable buddies for a limited time.