On the night of a crescent moon, veil-like wings dance across the sky…it would seem that Cresselia has been spotted! To fight Darkrai's nightmares together with everyone, Cresselia will appear in Pokémon Sleep soon! Pokémon Sleep has announced that Cresselia will begin appearing in the game starting on March 31st, 2025, at 3:00 p.m.

New Arrival: Cresselia

In certain areas, there have been reports of a Pokémon appearing that we haven’t seen around before—maybe this is thanks to the sleep research you’ve all been conducting! It may be possible to encounter it during sleep research sometime soon. We hope you’re looking forward to this!

New Arrival: Cresselia – Psychic

Newly Arriving Pokémon Details

Appearing Beginning: Mar. 31, 2025, at 3:00 p.m.
Note: Pokémon Incense for the new arrivals can be used starting from the date and time above.

Appearing at: Greengrass Isle, Snowdrop Tundra, and Lapis Lakeside
Note: Cresselia may still appear during regular sleep research even after the Cresselia vs. Darkrai event ends—but extremely rarely.
Note: The areas where Cresselia appears will differ from where it appears during the Cresselia vs. Darkrai event. Please see the event news for more details.

Sleep Type: Cresselia: Snoozing
Main Skill: Lunar Blessing (Energy for Everyone S)
Restores some Energy to all helper Pokémon on your team. Also gets some of each of the Berries the Pokémon on your team (including Cresselia) collect.
Note: The more different species of Psychic-type helper Pokémon on your team, the more Berries will be collected.

Example 1: A team with five different species of Psychic-type Pokémon
• Cresselia, Munna, Musharna, Espeon, and Gardevoir.

Example 2: A team with four different species of Psychic-type Pokémon
• Cresselia, Munna, Espeon, Espeon, and Gardevoir.

Special Pokémon

Cresselia is a Special Pokémon, and you can have only one Special Pokémon on your team.
Note: It’s possible to befriend more than one Special Pokémon, but you cannot have more than one on the same team. For each species of Special Pokémon, the Nature and sub skills will be identical for the first one befriended by each researcher.

Source: https://www.pokemonsleep.net/en/news/323436343539313738333736323332393631/