"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."
— Nightopians, according to the now unavaliable NiGHTS into Dreams web page
During gameplay, the Nightopians are spreaded through the levels where they just find ways of having fun and reproduce by dropping eggs around. If a Nightopian comes close to a Nightmaren, the Nightmaren will try to eat it.
The player can change the Nightopians mood by making them happy, sad, scared or angry depending on how NiGHTS treats them. In NiGHTS into Dreams, the levels music will change depending on the Nightopians mood.
Opening eggs in front of Nightopians or saving them from Nightmarens will make the Nightopians happy, when replaying the levels, they'll see NiGHTS as a friend and wave at him.
Drill dashing into a Nightopian will make them fear NiGHTS, when replaying the level, they'll run away scared of NiGHTS when they see him again. Paralooping a Nightopian in NiGHTS into Dreams and Christmas NiGHTS into Dreams will kill it, making the other Nightopians sad, scared or angry at NiGHTS.
In Christmas NiGHTS into Dreams and in the remasters of NiGHTS into Dreams released for PlayStation 2, PlayStation 3, Xbox 360 and PC, there's an unlockable feature known as the Nightopian Collection where the player can view their Nightopian information stored in a saved game file.
In every game, the Nightopians start disappearing after a while, starting from the oldest ones.
See main Article: MyDream/Nightopian Garden
In NiGHTS: Journey of Dreams, there's a feature called My Dream (Also known as Nightopian Garden). Unlike in the previous games, paralooping a Nightopian in this game won't kill it, instead, it will teleport the Nightopian to a garden where the player can take care of them. The player can also teleport Nightmarens and take care of them as well as feed them with Nightopians.
Mepians are a special type of Nightopian. They are Nightopians with Nightmaren parts. When NiGHTS attacks a Nightmaren, the nightmaren will turn into an orb and if the orb collides with a Nightopian, an egg will appear, when NiGHTS open it, a Nightopian with nightmaren parts will be born.
There are many types of Mepians for a single Nightmaren, they can be born with the Nightmaren's head, body, hands/claus and more. Hitting a Mepian with a different Nighmaren will make another Mepian that will have parts of both the hitten Mepian and the Nightmaren.
Each type of Mepian has an unique behavior, much like the regular Nightopians, NiGHTS behavior will affect their mood and they may befriend NiGHTS or hate him. Unlike the regular Nightopians, who always care about others of their kind, the Mepians may be nice or may be mean towards regular Nightopians, some Mepians can be agressive towards them and at the same time, they can befriend NiGHTS. It appears that Mepians with the head of a nightmaren are in particular the most agressive ones. The Mepians behavior will also affect the Nightopians mood, so if Mepians are mean towards them, the Nightopians will get sad or scared. Much like a regular Nightopian, the Mepians are Nightmarens prey, and may also be eaten by Nightmaren, they also disappear after a while.Examples of Mepians:
King PianThe King Pian (or Super Pian) is a rare type of Nightopian. As the name suggests, he is the king of all the Nightopians. When he appears, the King Pian will build a castle in the level, help the player keeping the Nightopians happy and attack the nightmarens by throwing rocks at them. If the Mepians are too agressive towards Nightopians, the King Pian will see them as a thread too and will attack these Mepians until NiGHTS get ride of them. Much like the other Nightopians, the King Mepian also disappears after a while, however, it takes much longer. The castle he builds is permanently added to the level though.
To acquire one, it's necessary to make two Mepians created out of two different Nightmarens breed. However, the Mepians can't have the same nightmaren body parts (example: if one Mepian has a nightmaren head, the other can't).
It should be noted that the chances of acquiring a King Pian are random, meaning breeding two different Mepians created from two different Nightmarens may simply result into another Mepian. The chances of getting a King Pian egg are 1 out of 4.
- Chrtimas NiGHTS into Dreams: Nightopians appear in Christmas NiGHTS into Dreams, where they have the same function as in the main games, except this time, they are wearing Christmas outfits.
- NiGHTS: Flying Through the Sky Without Wings: In the NiGHTS Storybook, Nightopians make minor appearance, first, watching NiGHTS playing his flute and later getting attacked by Nightmarens.
- NiGHTS into Dreams Archie Comics: Nightopians make minor appearances in the NiGHTS Archie Comics series. Two Nightopians called Napp and Snuze are major characters who helped NiGHTS after he started his uprising.
- Game Gag 1P Comic Sega-hen: Nightopians are one of the many NiGHTS characters to make a small appearance in the manga.
- Sonic Adventure and Sonic Adventure DX: A Nightopian made a cameo in Sonic Adventure and Sonic Adventure DX as a card in the NiGHTS thematic pinball table from Casinopolis.
- Shenmue: A Nightopian appears in Shenmue as a capsule toy next along with a Chao from the Sonic series.
- ChuChu Rockets!: After beating all of the 50 courses in ChuChu Rockets! The Nightopians are gonna be unlocked and it'll be possible to replace the ChuChus with them.
- Sonic Pinball Party: Nightopians appear in all of the NiGHTS thematic pinball tables in the game.
- Sega SuperStars Tennis: Nightopians appear during NiGHTS all star move and at the NiGHTS tennis court.
- Phantasy Star Portable 2: There's an unlockable Nightopian costume in the game.
- Sonic and All Star Racing Transformed: A Nightopian appears driving NiGHTS shapeshifted as a car and a lot of Nightopians can be seen through the Dream Valley circuit. Additionatly, a NiGHTS sticker set, which included a Nightopian sticker, was made for the game, but it got disconsidered. 
- 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.
- Sonic Runners: A regular Nightopian and a Nightopian wearing a christmas outfit appeared as an unlockable buddies for a limited time. The Nightopian may destroy all iron balls, spikes and enemies on screen when the player gets damaged, the Christmas Nightopian may trigger the Yellow Drill effect at random after a X combo.
- The Nightopians were designed after the Coneheads, a race of aliens that used to appear in early japanese advertisements for the SEGA Saturn in 1994.
- The Nightopians would later be the main inspiration for the creation of the Chaos from the Sonic series.
- According to the NiGHTS into Dreams manual, it was a Nightopian who found Elliot and Claris and introduced NiGHTS to them, though that is never shown on screen, this particular Nightopian is also never seen in the game.
- Weirdly, according to the game's manual, the Nightopian is said to have escaped from the dream world to go after Elliot and Claris, that contradicts with the game, where Elliot and Claris are seeing going to the dream world after running away from the Nightmarens.
- In the NiGHTS Archie Comics, two Nightopians called Napp and Snuze help NiGHTS after he starts his uprising, Snuze also goes to the waking world, meets up with Elliot, explains to him NiGHTS situation and leads him to NiGHTS. However, it's unknown if either Napp or Snuze was this particular Nightopian mentioned in the manual.
- In NiGHTS: Journey of Dreams, Owl has a similar role.
- In NiGHTS into Dreams, it's possible to acquire more than one Super Pian in the same level.
- The Nightopians are one of the seven NiGHTS characters who has appeared in every NiGHTS game, the others are NiGHTS, Reala, Wizeman, Elliot, Claris and Shleep.