You have to follow a distinct day and night cycle in Pokémon Sword and Shield. It only happens in the Wild Area in the game. Cities and routes are typically day time unless it changes for story reasons.
When you enter a city, the lighting in it may look different from other towns. For example, when you visit Turffield, it seems like later in the day, right before the evening. But it’s still bright out. You want to know about this because certain Pokémon evolve into something specific when you relate it to the time of day.
Eevee, for example, evolves into an Espeon if you have enough happiness with it, and it’s during the middle of the day. If, however, you level it up with the specific levels of happiness at night, you get an Umbreon. You want to make sure you level it up at the correct time so you can turn your desired Pokémon into the evolution form you wish to use. The same goes for Pokémon who prefer showing up at night.
So, while cities and routes remain attached to story elements to change their lighting, the Wild Area leans on your Switch’s internal clock. So if you want to go out and evolve a particular Pokémon at night, visit the Wild Area, and the same goes for when you want to evolve a Pokémon during the day.
You have the option to manually go in and modify your Switch’s clock to change the Wild Area’s cycle to better suit your needs. You can stand in the middle of the Wild Area and freely adjust these settings. To make sure you go after any Pokémon who do show up at particular times of the day, go to one area and then come back to reset the spawns.