Which Pokémon game first introduced the series’ day-and-night cycle?

Pokémon Gold and Silver first introduced the Pokémon series’ day-and-night cycle.

Released for the Game Boy Color in Japan in 1999 and internationally from 2000, the games used the system’s internal clock to divide gameplay into morning, day, and night. Time affected which Pokémon appeared, how some characters behaved, and when certain events could occur.

Gold and Silver also introduced a real-time week structure, a wider roster of Pokémon, breeding, held items, and the Johto region. These additions made the games feel like a living continuation rather than a simple remake of the first generation.

The cycle is sometimes confused with the clock feature in later games, which expanded the system with more detailed schedules and seasonal mechanics. The original landmark, however, belongs to Gold and Silver, making them the correct pair for this question.