Trainers using Pokémon Sleep are able to catch Shiny Pokémon alongside their Sleep Styles study mission from Professor Neroli. Like most Pokémon games, like the connected Pokémon GO, Shiny Pokémon are special alternate colored creatures that are lucky to come by. The wonderful difference in this game however is that in order to find and catch them, players need to track their sleep. Instead of a long grind to for an encounter, simply stick to the tips and tricks to encounter one in order to achieve them faster.

https://screenrant.com/pokemon-sleep-how-to-catch-shiny-hunting/