1. Sleep apnea is a condition that people stop breathing
when they sleep.
2. Sleep apnea is a condition in which people stop breathing
when they sleep.
Which sentence is correct? Thanks a lot.

Sentence 2 is the correct one.

Sleep apnea is that people stop breathing up to 30 or 40 times an hour during their sleep.
Is this sentence grammatical and meaningful? Thanks again.

Sleep apnea is that people stop breathing up to 30 or 40 times an hour during their sleep.

Is this sentence grammatical and meaningful? Thanks again.

Awkward syntax. Recast as:

Sleep apnea is a condition in which people stop breathing up to 30 or 40 times an hour during their sleep. Or "while they sleep."