Every a-life has a mood value which can range from 0 to 3. A-life's mood affects the music and how it responds to your presence. If the a-life's mood is 3, it will smile and wave when close to the player, and will follow them around. If mood is 2, the a-life will ignore the player's presence. If it's 1 or 0, it will flee from the player. The starting population of nightopians all have 2 mood, and there are several conditions that can modify this value over time.
Paralooping a-life has a negative effect on mood. Specifically, if the one you paralooped was interacting with another a-life, the surviving partner's mood will immediately drop to zero. For nightopians only, paralooping 3 or more nightopians in a single playthrough instantly lowers all surviving nightopians' moods to 0.
New eggs will inherit the mood of one of their parents, while hatching the egg sets the newborn a-life's mood to 3.
Mepians have one behavior that judges the player's performance during acrobat time. The way this behavior is intended to work is that doing 2 or fewer tricks will decrease their mood by 1, performing 3 or 4 keeps it the same, and doing 5 or more raises their mood to 3 immediately.
However, the code contains a flaw that causes the mood update to spill over into an adjacent value, which can result in underflow when 1 is subtracted from it.
Both types of a-life have a behavior where they can talk to each other. If this behavior is called twice in a row, there's a possibility for one a-life's mood to be transferred to the one it's conversing with. This will only occur if one a-life's mood is 2 (neutral) and the other's mood is not neutral. The result is that the non-neutral mood is transferred.
It seems that the talking behavior is never called twice in a row in practice, which means mood probably can't be transferred using this method. More research is needed.
When a nightopian moves near a nightmaren or mepian, there is an opportunity for a nightopian's mood to change based on the player's action towards the hostile. If the player touch dashes it toward the nightopian (only possible with nightmarens), the nightopian's mood will drop to 1. If the player paraloops the hostile instead, the nightopian's mood will increase by 2, capped at 3.