Among those questions, Nick from Trainer Tips (check out the video below later) voiced the most interesting: does time of day influence spawn frequencies of Pokémon Types or specific Pokémon species?
It turns out that Nick was on the right track, as the results we observed in our initial research are actually consequences of something else: nocturnal specie exist in Pokémon GO!
Research and results
The starting point of this research is a table of nocturnal Pokémon in the main game series. The list is available on Bulbapedia. We included both Generation I and Generation II Pokémon in the research, with hope to identify all nocturnal spawns.
We are happy to announce that the following specie are more likely to spawn at night than during the day:
As you can see above, several Pokémon from the Bulbapedia list do not appear to be nocturnal specie in Pokémon GO. Even further, some evolved or pre-evolution forms of confirmed Nocturnal Specie are also not nocturnal in Pokémon GO!
Prime examples of the latter are Spinarak and Wooper. Spinarak proved to spawn throughout the day, while Airados is more likely to spawn during the night. Same goes for Quagsire and Wooper. Data for Persian and Gloom was not conclusive.
It is apparent that some Pokémon GO specie are nocturnal specie, with higher chance to spawn during the night than during the day. The night cycle lasts from 20:00 local time to roughly 08:00 local time.
Our initial research (“Analysis of how time of day influences different Pokemon Type spawns”) indicated that those specie are of DARK and GHOST type, however, it seems that our initial conclusions were not completely correct.
Although some nocturnal specie in Pokémon GO are DARK or GHOST types, there are several others that spawn more often during the night and have other Pokémon Types.
The data set used for analysis is published on GitHub, so refer to this link for the complete data set.