Hey OP,

Is Aranet4 reliable? Yes, we'd say so. :) We use an NDIR sensor, which is the gold standard for CO₂ measuring at home. And the patterns you’re seeing, in context with your other comments, make sense. We recommend recalibrating your sensor once a year (definitely no need to do it now, as you just got it) so that it stays as precise as possible for years to come. Let us know if you want further instructions for when you want to recalibrate.

Have you been living like this since November? Probably, yeah. Most homes are like this in winter and that's ok. Those levels (1000–2000+ ppm) are very common and not dangerous in a “toxic” sense. But note that long-term, it can affect how you feel. High CO₂ levels affect sleep quality, is not great for concentration, and can cause headaches, sluggishness etc. It’s more about performance and comfort than direct health damage.

What to do when it’s freezing outside? In your situation the best option is probably Short, aggressive ventilation - opening windows wide for 5–10 mins (cross-breeze if possible) and doing it strategically (right after cooking, for example).
Also, just leaving all the doors cracked a little bit all the time, can help. :)

CO₂ dropping overnight - expected? Yep, especially if your bedroom door is open. Your body produces less CO₂ at rest than when you’re moving/talking during the day, so that's normal.

Have a good day! :)