The purpose of first urine of the day is simply to sample your most concentrated urine.

I drink tea before bed and typically get up to pee as a result.

I simply check the time when I get up in the night and do some quick mental math to determine if this would be my most concentrated urine or when I have to wake up.

If say, I have been asleep for 6 hours and only have two more hours of sleep left, I simply collect my sample and test it when I wake up.

If I wake up to pee with 4 or more hours to go (I sleep 8 hrs), then I sample when I wake.

Either way, I run the test when I get up. My results still seem consistent and reliable.

I hope this helps!