daytime, daylight
晝 centers on the bright part of the day: daytime and daylight, the period when the sun is up and things are visible.
晝 is the traditional form of 昼. It combines 聿 (a hand holding a brush, suggesting drawing or marking) with 旦 (sunrise), likely expressing the idea of marking the boundary of daylight. The modern simplified form 昼 replaces the top with 尺.
The top 聿 looks like a hand holding a brush, and the bottom 旦 shows the sun rising above the horizon. Picture someone painting the bright daylight sky: 晝 is daytime.
For チュウ, imagine the sun at high noon: the sound of 'chew' as you chew your lunch during the daytime. Chew -> チュウ, and the sun is directly overhead.