You must follow a distinct day and night cycle with Pokémon Sword and Shield. It only happens in the wilderness area of the game. Cities and routes are usually daylight, unless it changes for history reasons.
When you enter a city, the lighting may appear different from other cities. For example, when you visit Turffield, it feels like later, right before the evening. But it is still clear. You want to know why some Pokémon evolve into something specific when you link it to the time of day.
Eevee, for example, evolves into an Espeon if you have enough happiness with it, and that's during the middle of the day. If, however, you level up with specific happiness levels during the night, you get an Umbreon. You want to make sure you level up at the right time so that you can transform your desired Pokémon into the evolution form you wish to use. The same goes for Pokémon that prefer to show up at night.
So while cities and paths remain connected to story elements to change their lighting, the Wild Area rests 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 when you want to evolve a Pokémon during the day.
You have the option to manually log in and change your Switch's clock to change the Wild Area cycle to best suit your needs. You can stand in the center of the wilderness and freely adjust these settings. To make sure you follow any Pokémon that show up at particular times of the day, go to an area and then return to restore spawns.